su $USER -c "rm ..." not working from gdm script

["Robert C. Mosher II" <mosher@andrews.edu>]

I am trying to delete a file created by a user in their home directory
through the gdm PostSession script. This is proving difficult since
/home is an nfs mounted directory. This is the line of the script. 

su $USER -c "rm -f /home/$USER/.smb.lock" 

I know $USER is correctly set, and this script works when run from the
command line. I also know that this line of the script is reached,
though I haven't found any error messages generated. Any ideas why this
would not be working?

Thanks,
Rob Mosher