Probably your item is a lot/serial controlled one and you need to enter those values while delivering.
Below are my guesses.
a) Item remains in Receiving.
b) The status tab in the Purchase Order Shipments still shows quantity received.
c) The item is not lot controlled.
If above guesses are true, this is the classic case where you would need to raise an SR. I faced a similar problem and have been patched once.
Thanks for the information above but this issue was due to the item not being set to Active, soon as i changed this i was able to deliever the transaction into the sub inventory.
In this case this was a functional setup issue which i missed.