Not able to add some items to P.O.'s

Since I have added the latest SP I am not able to add some items to P.O's. Ihave checked that the vendor's match and even tried all vendors. The only way I can see the item is if I use the "Find" feature. But even then the item will not be added to the "Add Items" Screen. The only way I am able to add items is to use the "quick scan". Is this a bug? I have tried reindexing my DB with no results.

Thanks