Excel macros run through powershell but not when run by windows task manager

I have a script which checks a folder for excel files and then if this "threshold" is greater than 0 then a macro from another excel file is run that interact with these excel folders.

When I run the process manually through powershell ISE it works fine but when I use the windows task scheduler the powershell script runs but the excel macro called doesn't run. Any suggestions why this might be the case? This process used to run on a windows 2008 server fine but was migrated to windows server 2012 and won't run properly

  if ($count -gt $threshold){
    $excel = new-object -comobject excel.application

    $workbook = $excel.workbooks.open("D:\TimesheetService\IS-FS - AutoTimesheetLoader v2.3 - UAT.xlsm")

    $worksheet = $workbook.worksheets.item(1)

    $excel.Run("ImportTime")

    $workbook.close($false)

    $excel.quit()
    [System.Runtime.Interopservices.Marshal]::ReleaseComObject($excel)
    Remove-Variable excel
}