Encoder Counts

Hi,
I am using Siemens CPU 314C with encoder counts. I am also using input I1.4 for zero channel latch value. Encoder is fitted on roller and second roller have one proxy switch for latch detection. i want to calculate difference of count value after every proxy latch pulse. but i am not able to get difference value. some time i get difference and some time get zero. Please help me. Please see the attached picture let me know where am i wrong.
Best Regards,

For some reason, the attached image doesn't pop up for me and doesn't have great resolution.

TRY This.