Merge-SPLogFile not working in Powershell

It is likely that your TimerJob-Instances are down. Please check with the following script:

$farm = Get-SPFarm
$FarmTimers = $farm.TimerService.Instances
foreach ($FT in $FarmTimers)
{
write-host "Server: " $FT.Server.Name.ToString();
write-host "Status: " $FT.status;
write-host "Allow Service Jobs: " $FT.AllowServiceJobs;
write-host "Allow Content DB Jobs: " $FT.AllowContentDatabaseJobs;"`n"
}

Use the following Script to start disabled Instances again:

$farm = Get-SPFarm
$FarmTimers = $farm.TimerService.Instances
foreach ($FT in $FarmTimers)
{
write-host "Server: " $FT.Server.Name.ToString();
write-host "Status: " $FT.status;
write-host "Allow Service Jobs: " $FT.AllowServiceJobs;
write-host "Allow Content DB Jobs: " $FT.AllowContentDatabaseJobs;"`n"
}
$disabledTimers = $farm.TimerService.Instances | where {$_.Status -ne "Online"}
if ($disabledTimers -ne $null)
{
foreach ($timer in $disabledTimers)
{
Write-Host -ForegroundColor Red "Timer service instance on server " $timer.Server.Name " is NOT Online. Current status:" $timer.Status
Write-Host -ForegroundColor Green "Attempting to set the status of the service instance to online..."
$timer.Provision()
$timer.Start()
write-host -ForegroundColor Red "You MUST now go restart the SharePoint timer service on server " $FT.Server.Name}}
else
{
Write-Host -ForegroundColor Green "All Timer Service Instances in the farm are online. No problems found!"
}

Reference: SharePoint Timer jobs not running(Specially one-time timer jobs)

There could be couple of things which can cause this issue.

• check if SharePoint tracing service is running on the server.
• check if SharePoint Admin service is running on the server.
• Also check from central admin, if their is any time job name Collection of log files from server "servername" stuck. CA > Monitoring > Running job status
• You may need to clear the Config cache to get it working.
• I would also check the event log and see if there is any other clue, may be it is permission issue.

Side note, if it is single server farm, and if you know the timing of the correlation id, You can simply get the log file which contains that timing and open it in ULS viewer and filter that file with your correlation id.