There are some reasons.
It depends on the version of MFG how to solve them.
The first is, can you check that ALL items have been processed. Check the logging of the full mrp run.
One of them is to recalculate the Low Level Codes of ALL items.
Do users modify products during the MRP run?
The following actions modify the MRP-required flag.
- All inventory related actions: purchase, sales, receipts, issues etc
- Modify an item field in the 1.4... , 13.. or 16.. menu
Another important one is that one of the parent items is not used during MRP-run because of the planning parameter setting in 1.4.7.