Sorting of purchase order positions

Hi Guru's!

In former times the positions of the purchase order were not really sorted.

But whenever a material was twice or more times within the PO, the these

positions were sorted by date.

Now I have the problem that the dates of more positions of one material

are disordered. Consequently we have problems with MIGO.

The first position of a material is posted in the goods receipt of purchase order and not the first date.

Has anybody an idea, if there is a note about that,

or how I can solve the problem.

Thanks a lot,

Barbara