You can monitor your shipment feeds at https://unity.scanpower.com/list/feed (you must be logged into ScanPower for this link to work).  When you click FINISH SHIPMENT three feeds are sent to Amazon (PRODUCT, INVENTORY and PRICE).  When the feeds are received successfully by Amazon you will Success for the corresponding feed in the STATUS column.  If the PRODUCT feed FAILS/HAS ERRORS but the INVENTORY and PRICE feeds are successful it is often due to item-specific issues and not that the entire PRODUCT feed failed.  You can monitor this by clicking the corresponding ID to the far left.  There you will see any item-specific error messages we received when we sent the PRODUCT feed.  Any items associated with these feeds will likely appear as stranded inventory in Seller Central.