Problem with One SU created from multiple at Goods Receipt

Hi, I have received one material which consisted of a number of SUs. The batch is the same for all  the material, and it is being stored in a bulk storage area which is a push back bin. The problem is that the new SU numbers  are not being created. One SU keeps getting bigger. For example 1 SU should be 1000 ea only but  now one SU is increasing quantity each time I receive another  and now has 4000 ea. No new SUs number are being generated. Anyone have any idea why this could be?  How can i stop this happening?