Amazon FBA shipment closed 0 out of 27 items received

Because they messed up. But here's a warning. If you do not have invoices for those items DO NOT open up a case. I have experienced this. I, like you, had items received, checked in and then mysteriously disappear. I opened a case and they asked for invoices, I did not send them to them because I wasn't looking for reimbursement, I wanted them to find my inventory. But for some reason, they need invoices to look for items they checked in to their warehouse. About 2 months later I awoke to those items in my inventory. Sold about 3 items. But the very next day they were missing again. I go to the inventory ledger and its shows the items were removed due to ownership issues. They knew they belong to me because of the seller sku so they put them in my inventory. But because I never sent them the invoices they said the items weren't mine. If I had never opened the case they would have just placed them back in my inventory, where they would have stayed. Lesson learned.