I think your PO item needs to be received in full rather than partially. Are you sending a quantity that is less than the requested PO item quantity?
If that is true, then you need to ask the business/functional folks whether this situation can happen in real life. If they say that is how it happens in reality, then you need the functional folks to change the config to allow partial/multiple deliveries against a PO line item. If they say no, that should not happen, then you have to always validate before calling the BAPI to make sure that the PO line item quantity always equal to the GR quantity.
Another thing to remember is that sometimes there are over delivery and under delivery tolerances defined in config. Also, if info records are there, then they also control how the GR validations are performed. Also check if you have to mark the delivery complete indicator on the item.
Srinivas
