When you create a shipment in ScanPower, and add listings, partial feeds are sent to Amazon so Amazon can check the item against its current inventory in order to assign the items to the most appropriate Fulfillment Center (FC). The final feed goes over when you click FINISH SHIPMENT.  Because of this, it can be confusing to toggle between ScanPower and Seller Central while creating a shipment.

Once you click FINISH SHIPMENT and send your items to Amazon the listing will go live once Amazon has received the item and scanned it into their inventory.  It can some times take up to 24-hours after clicking FINISH SHIPMENT for your items to appear in Seller Central with complete listings as Amazon needs to process the feeds.