Wrong calculation in Requisition Worksheet

paul_van_rossum

I encounter the following problem in Attain 3.60 when working with requisition worksheets:
I first created a Production Forecast with quantities 100, 200 and 300 in resp. april, may, and june 2005. My Reordering Policy is Order, all other flags set to Yes, no Reorder Point or Minimum stock registered.
When I first run the Calculate Plan it gives me 3 new action messages:
New 250 per april 1st
New 250 per may 1st
New 250 per june 1st
This is OK, so I carry out the action messages to create the PO.

When I now rerun the Calculate Plan (without changing anything in the data) it gives me the following results:
cancel 250 per april 1st
cancel 250 per may 1st
cancel 250 per june 1st
New 250 per april 1st
New 250 per may 1st
New 500 per june 1st.

Why do I have now a quantity of 500 in June? My requirements haven't been changed, so why does the quantity change?
If I remove the purchase order and then re-run the Calculate Plan it gives me the correct quantities again, so it must have something to do with the purchase order interfering with net demand.

Thanks for any help on this