Problem with background GPS

[Kikka]

Hi Gurus,
I have a problem regarding a planning function execution after saving data in a layout.
When I save some data in a layout it automatically starts a job with a GPS that has two planning functions.
Those functions have to read saved data , calculate and write new data.
This procedure works fine, but sometimes the job gets completed in 3 seconds and it doesn't write anything.
I don't understand why this happens. We work with sap bw 3.5. Colud be a problem with buffer?
Can you help me?

Best Regards
K.