Missing inbound FBA shipments

I need advise on how to resolve the issue I am having with my FBA shipments. I sent three shipments between June and July and each of the shipments was about 3-5 boxes and contained the same items. The weights of the packages I imputed during the FBA shipment creation on Amazon’s website were exactly as stated by UPS (Amazon Partnered Carrier) on the website will ALL boxes marked as delivered. Over 300 units are missing from all three shipments combined and the shipments are now showing as closed. I contact seller support to be compensated for the missing units and my cases have been closed several times with no reimbursements for the lost items. Can someone please shed some light here on what my next step should be as it is unacceptable for this to go unchallenged.

Thank you