NT BACKUP Desides when it will work

I have 2 clients with the exact same problem. I created a cmd file to run nt backup and then I have that file scheduled to run at 11pm using the scheduling service. My problem is that everything works great for about 2 weeks then stops working. When I show up onsite and check the event viewer there are no error messages. Then when I try to launch nt backup I get and hourglass for about 3 seconds and then nothing happens. I check the task manager and is does not show NT Backup Running. If I restart the server the backup will work fine for another 2 weeks but then fail again in the same way. Any Ideas?