problem with sorting Quote

I have following custom quote page and controller but the problem is sorting is not working properly, When i click on column for sorting ascending and descending it will not show me the list with sorted and automatically hidden the list of quote,

Visualforce Page :

<apex:page controller="MyPagingController" tabStyle="Quote"> 
<apex:sectionHeader title="List of Quotes"></apex:sectionHeader> 
<apex:form > 
    <apex:pageBlock title="Search Quote"> 
        <apex:outputLabel style="font-weight:Bold;" value="Quote Name : " /> 
        <apex:inputText value="{!searchQuoteName}" /> 

        <apex:outputLabel style="font-weight:Bold;" value="Quote Number : " /> 
        <apex:inputText value="{!searchQuoteNumber}" /> 
        <br/><br/> 

        <apex:commandButton value="Search" action="{!Search}" reRender="pageBlock"></apex:commandButton> 
    </apex:pageBlock> 
</apex:form> 
<apex:form > 
    <apex:pageBlock title="List of Quotes" id="pageBlock"> 
        <apex:pageBlockButtons location="top" dir="LTR" > 
            <apex:commandButton value="Create New Quote" action="/apex/QuoteGroupCreate?editmode=False" /> 
        </apex:pageBlockButtons> 


        <apex:pageBlockTable value="{!quotes}" var="c" rendered="{!NOT(ISNULL(quotes))}" rows="{!PageSize}"> 

            <apex:column width="15%"> 
                <apex:facet name="header"> 
                    <apex:commandLink action="{!ViewData}" value="Quote Name{!IF(sortExpression=='name',IF(sortDirection='ASC','▼','▲'),'')}" id="cmdSort"> 
                        <apex:param value="name" name="column" assignTo="{!sortExpression}" ></apex:param> 
                    </apex:commandLink> 
                </apex:facet> 
                <apex:outputlink value="/apex/QuoteDetail?id={!c.Id}">{!c.name}</apex:outputlink> 
            </apex:column> 

            <apex:column width="35%" headerValue="Opportunity Name"> 
                <apex:outputlink value="/{!c.OpportunityId}" target="_blank">{!c.Opportunity.Name}</apex:outputlink> 
            </apex:column> 

            <apex:column width="10%" value="{!c.QuoteNumber}"> 
                <apex:facet name="header"> 
                    <apex:commandLink action="{!ViewData}" value="Quote Number{!IF(sortExpression=='QuoteNumber',IF(sortDirection='ASC','▼','▲'),'')}" id="cmdNumberSort"> 
                        <apex:param value="QuoteNumber" name="column" assignTo="{!sortExpression}" ></apex:param> 
                    </apex:commandLink> 
                </apex:facet> 
            </apex:column> 

            <apex:column width="10%" headerValue="Quote Amount" value="{!c.GrandTotal}"/> 

            <apex:column width="15%" value="{!c.createdDate}"> 
                <apex:facet name="header"> 
                    <apex:commandLink action="{!ViewData}" value="Date{!IF(sortExpression=='createdDate',IF(sortDirection='ASC','▼','▲'),'')}" id="cmdCreatedAtSort"> 
                        <apex:param value="createdDate" name="column" assignTo="{!sortExpression}" ></apex:param> 
                    </apex:commandLink> 
                </apex:facet> 
            </apex:column> 

            <apex:column width="35%" headerValue="Created By"> 
                <apex:outputField value="{!c.createdById}"/> 
                <!--<apex:outputField value="{!c.createdDate}"/>--> 
            </apex:column> 

            <apex:column width="35%" headerValue="Modified By"> 
                <apex:outputField value="{!c.LastModifiedById}"/> 
                <!--<apex:outputField value="{!c.LastModifiedDate}"/>--> 
            </apex:column> 

            <apex:column width="8%" headerValue="View"> 
                <apex:outputlink style="color:blue;" value="/apex/QuoteDetail?id={!c.Id}">View Detail</apex:outputLink> 
            </apex:column> 

            <apex:column width="5%" headerValue="Edit"> 
                <apex:outputlink style="color:blue;" value="/apex/QuoteGroupCreate?id={!c.Id}">Edit</apex:outputLink> 
            </apex:column> 
        </apex:pageBlockTable> 
        <div align="right" style="display:{!IF(AND(NOT(ISBLANK(quotes)),quotes.size>0),'block','none')}"> 
            <br/> 
            <font size="1pt">Page #:&nbsp;<apex:outputLabel value="{!PageNumber}"/>&nbsp;out of&nbsp;<apex:outputLabel value="{!totalPageNumber}"/>&nbsp;&nbsp;&nbsp;&nbsp;</font> 
            <apex:commandButton value="Previous" action="{!previous}" disabled="{! NOT(hasPrevious)}" reRender="pageBlock" ></apex:commandButton> 
            <apex:commandButton value="Next" action="{!next}" reRender="pageBlock" disabled="{! NOT(hasNext)}" ></apex:commandButton> 
        </div> 
    </apex:pageBlock> 
</apex:form> 
</apex:page>

Apex Class :

public class MyPagingController{
    public transient List<Quote> quotes{get;private set;}

    private String sortDirection = 'DESC';
    private String sortExp = 'createdDate';
    public final decimal pageSize {get;set;}
    public Integer totalPageNumber {get;private set;}
    public String searchQuoteName { get; set; }
    public String searchQuoteNumber { get; set; }

    public ApexPages.StandardSetController setCon{get;set;}

    public Boolean hasNext {
        get {
            return setCon.getHasNext();
        }
        set;
    }
    public Boolean hasPrevious {
        get {
            return setCon.getHasPrevious();
        }
        set;
    }

    public Integer pageNumber {
        get {
            return setCon.getPageNumber();
        }
        set;
    }


    public void previous() {
        setCon.previous();
        quotes= (List<quote>)setCon.getRecords();    
    }

    public void next() {
        setCon.next();
        quotes= (List<quote>)setCon.getRecords();        
    }

    public String sortExpression    {
        get 
        {
            return sortExp;
        } 
        set
        {
            if (value == sortExp)
                sortDirection = (sortDirection == 'ASC')? 'DESC' : 'ASC';
            else
                sortDirection = 'ASC';
            sortExp = value;
        }
    }

    public String getSortDirection()    {
        if (sortExpression == null || sortExpression == '')
            return 'ASC';
        else
            return sortDirection;
    }

    public void setSortDirection(String value)    {  
        sortDirection = value;
    }

    public MyPagingController()  {
        pageSize = 20;
        string queryString = 'Select Id, Name, Opportunity.Name, QuoteNumber, TotalPrice, CreatedDate, GrandTotal, CreatedById, LastModifiedById from Quote';
        setCon = new ApexPages.StandardSetController(Database.getQueryLocator(queryString));
        setCon.setPageSize((Integer)pageSize);
        quotes= (List<quote>)setCon.getRecords();    
        totalPageNumber=(Integer)  (setCon.getResultSize()/pageSize).round(System.RoundingMode.CEILING);
    }

    public PageReference ViewData()
    {   
        BindData();        
        return null;
    }

    private void BindData()
    {
            string sortFullExp = sortExpression  + ' ' + sortDirection;            
            string searchQuoteName = searchQuoteName;
            String searchQuery = '';
            String s = '';
            String squote = '';

            if(searchQuoteName != null){
                s = '\'%' + searchQuoteName + '%\''; 
                searchQuery = ' WHERE Name LIKE '+s; 
            }

            if(searchQuery != ''){
                if(searchQuoteNumber != null){
                    squote = '\'%' + searchQuoteNumber + '%\''; 
                    searchQuery += ' AND QuoteNumber LIKE '+squote; 
                }    
            }else{
                if(searchQuoteNumber != null){
                    squote = '\'%' + searchQuoteNumber + '%\''; 
                    searchQuery = ' WHERE QuoteNumber LIKE '+squote; 
                }
            }

        string queryString = 'Select Id, Name, Opportunity.Name, QuoteNumber, TotalPrice, CreatedDate, GrandTotal, CreatedById, LastModifiedById from Quote '+searchQuery +' order by ' +sortFullExp+ ' Limit 10000';
        System.debug('query *******' + queryString);
        setCon = new ApexPages.StandardSetController(Database.getQueryLocator(queryString));       
        setCon.setPageSize((Integer)pageSize);
        quotes= (List<quote>)setCon.getRecords();        
        totalPageNumber=(Integer)  (setCon.getResultSize()/pageSize).round(System.RoundingMode.CEILING);
    }

    public PageReference Search() {
        BindData();
        return null;
    }
}