Siemens Traversing Block Problem

Dear Experts,
I wanted to discuss a problem which I am going through now and you experts can suggest me a solution for this.
I am using S120 servo's and using traversing to control the positions of the servo. The problem is every now and then, I am losing the Traversing Active signal from the drive and that cause no motions to servo. I can see that there is right traverse block number being fed to block plus also there is this bit activate Traversing Task which goes high but as I mentioned earlier, I am losing the Traversing Command active, which I believe comes from the drive status word.
Can somebody explains me how to trouble shoot this.

Many Thanks in Advance
Manoj

Too many things to guess...
Set up a trace for digital signals
I assume your not getting any alarms? it just stops?
Are you calling the tasks from the plc? (I see your using 2091.x for traversing block selection)

Thanks JRW for responding.
The traverse move number is active via 2091.x and also there is no alarms in the drive. Yes you are right, I am calling the tasks from the PLC.
I will give a try with Trace and see if that works.
Will post the outcome of it.

Regards
Manoj