Android button not visible in scrollview

In my android project I have xml file below:--

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:gravity="center_vertical|center_horizontal"
    android:orientation="vertical" android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin" tools:context=".LoginActivity"
    android:background="#ffBE80FF"
    android:descendantFocusability="beforeDescendants"
    android:focusableInTouchMode="true">

    <LinearLayout
        android:id="@+id/ll1"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:gravity="center_vertical|center_horizontal">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textAppearance="?android:attr/textAppearanceLarge"
            android:text="FOOBAR"
            android:id="@+id/textView2"
            android:textSize="30dp"
            android:textColor="#ffffffff" />


    </LinearLayout>

    <LinearLayout
        android:id="@+id/ll2"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:gravity="center_vertical|center_horizontal">
        <ProgressBar android:id="@+id/login_progress" style="?android:attr/progressBarStyleLarge"
            android:layout_width="wrap_content" android:layout_height="wrap_content"
            android:visibility="gone" />
        <ScrollView android:id="@+id/login_form" android:layout_width="fill_parent"
            android:layout_height="fill_parent"
            android:fillViewport="true" >
            <LinearLayout android:layout_width="match_parent" 
                android:layout_height="wrap_content"
                android:orientation="vertical">

               <!--  <LinearLayout android:id="@+id/email_login_form"
                     android:layout_width="match_parent"
                    android:layout_height="wrap_content" android:orientation="vertical"> -->

                    <AutoCompleteTextView android:id="@+id/email" android:layout_width="match_parent"
                        android:layout_height="wrap_content" android:hint="@string/prompt_email"
                        android:inputType="textEmailAddress" android:maxLines="1"
                        android:singleLine="true"
                        android:layout_marginBottom="10dp"
                        android:textColorHint="#ffffffff"
                        android:textColor="#ffffffff" />

                    <EditText android:id="@+id/password" android:layout_width="match_parent"
                        android:layout_height="wrap_content" android:hint="@string/prompt_password"
                        android:imeActionLabel="@string/action_sign_in"
                        android:imeOptions="actionUnspecified" android:inputType="textPassword"
                        android:maxLines="1" android:singleLine="true"
                        android:layout_marginBottom="10dp"
                        android:textColorHint="#ffffffff"
                        android:textColor="#ffffffff" 
                          android:layout_weight="0.5"/>

                    <Button android:id="@+id/email_sign_in_button" style="?android:textAppearanceSmall"
                        android:layout_width="fill_parent"
                        
                         android:layout_height="wrap_content"
                        android:layout_marginTop="16dp" android:text="@string/action_sign_in"
                        android:textStyle="bold"
                        
                        android:textColor="#ffffffff"
                        android:background="#ff7e51c2" 
                          android:layout_weight="0.5"/>

                <!-- </LinearLayout> -->
            </LinearLayout>
        </ScrollView>
    </LinearLayout>

    <LinearLayout
        android:id="@+id/ll3"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:gravity="bottom|center_horizontal">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Sign up"
            android:id="@+id/signUpTextView"
            android:autoLink="web"
            android:textColor="#ffffffff" />
    </LinearLayout>
</LinearLayout>

Its displays properly what I need.It looks like below picture:-

But When I want to fill up the form.that time log-in button is not visible. its looks like below picture:-

What am I doing wrong in my code?? Why it is not visible when i want to fill up the edit-text fields? please guys suggest me.