Task Scheduler not running jobs

I have installed Veritas Backup MyPC on a workstation.
I have scheduled a number of backups to run at different times. The scheduler uses the one that comes with Windows 2000.
Here is my problem.
The tasks donot run as scheduled all the time. IfI go into the scheduler in the morning and try to manually run the task nothing happens.
I then restart the workstation then go back into the scheduler and manually run the task and it works. The backup runs and averthing is OK.
So what is causingthe Task Scheduler in Win 2000 to not run tasks?