Directory object not found error in Get-ADGroupMember

Question

I am new in PowerShell scripting. I am writing a script to add users in different AD Groups. While doing so I do the following:

1. Check if the user already exist in the group:

    $mbr_exist = Get-ADGroupMember $grpname | Where-Object {$_.SamAccountName -eq $sam}

2. If user does not exist, then add the user to the group.

When I manually run the script it runs flawless without any errors. But when I schedule the script to run it gives an error as follows:

3/30/2015 8:32:15 AM **Directory object not foundAt** 
+ $mbr_exist = Get-ADGroupMember $grpname | Where-Object {$_.SamAc ...
+                              ~~~~~~~~~~~~~~~~~~~~~~~~~~ 
Error at Line:$mbr_exist = Get-ADGroupMember $grpname | Where-Object {$_.SamAccountName -eq $sam}

The strange thing is the user for which it throws the error is present in the group. I am not sure why this error is occurring when scheduled.

$grpname is defined as $grpname += "Group1" ' it's a array of strings (No Blanks spaces). Added a for loop to access one group at a time.