Make sure that the file extension is .tcl and look for the text "Backup has been scheduled." in your log file.
If you don't have that in your log file try adding "ns_log notice {beginning backup script}" at the beginning of that file and "ns_log notice {completed backup script}" to the end of it and look to see if AOLServer is starting to source your script and not finishing, or not even starting.
What version of OpenACS are you running?