MRP keeps pushing delivery schedules out, refusing to firm them (ME33L)

I hope someone has seen something like this before and can let us know where to look. MRP keeps pushing our delivery schedules out even though it is recommending that they be rescheduled in. What we'd like to happen is for MRP to leave these schedules alone so that they would move inside the time fence and be firmed automatically. Here is what is happening:

Part X, MRP type P1, firming type 1, MRP group blank. Planning time fence 05/07/2007, available quantity -20.

Three schedule lines exist for 05/07/2007, two for 10 and another for 30.

MRP will run tonight. Processing Key = NEUPL, purch req = 2, sched lines = 3, MRP list = 1, planning mode = 2, scheduling = 2

Tomorrow, the planning time fence will be at 05/08/2007 and the three schedule lines will move out to 05/08/2007. The won't move inside the planning time fence and firm up. They SHOULD BE 05/07/2006 and firmed!

We originally thought that we may have to create a release with ME84 before these would firm, but it made no difference. They still won't firm unless someone manually changes the date on each and every schedule line.

Does anyone know what we can change or run to get these schedules to firm up instead of having them pushed out continually?