Scheduled backups are not running

I have just migrated our SharePoint farm to new servers in a new datacenter.  Backups ran fine on the previous WFE but not on the new one.

The backup script that runs fine when I run it from SharePoint Management Shell but not from task scheduler.

The task history in task scheduler shows that the task was completed with Operational Code (2).

This is the action defined for the task in the scheduler:

Action: Start a program

Program/script: C:\windows\system32\WindowsPowerShell\v1.0\powershell.exe

Arguments: -command "& C:\SharePoint - Custom\BackupFarm.ps1"

For testing I have it set to run as me, Run whether user is logged on or not, and Run with highest privileges.

Thanks to anyone who can help me out.

David