NTBACKUP won’t start

Hye, I have a problem with my windows NT server 4.0 spack 4.0, My backyp is suppose to be started every day by winat command. My tape machine is o.k., tapes also but the windows nt backup program won’t start. It is there in the process list of the task manager but I can’t kill it.
I need to reboot my server in order to get it bak on but the next backup scheduled it won’t start again…
What is happening and what to do to solve it !!
Thank’s in advance …