Missing Items from Inbound

So I send a shipment in to FBA a small parcel as usual, I do this 2 or 3 times most weeks, its a small parcel 14 SKU's containing a total of 298 units. Four days later its checked in as usual but then 15 days later its still checked in and not moved to receiving, so I create a SS case, asking them to check this for me as I had sent 5 more shipments in after that were being received as normal. Got the brush off - you just need to wait, no interest in helping whatsoever so I leave it another week, still the same so I contact them again- same result, you need to wait then you can ask for it to be investigated.

Weeks later shipment closes no units received so I do my work send all the documents in and launch an investigation, 3 days later they deny my claim stating

Hello from Amazon Selling Partner Support

As you requested, we have completed the review of shipment FBA15########

We are denying your reimbursement claim for the following units because we received an unexpected item when compared to your shipping plan. This was caused by sending items with the wrong UPC, or items having missing or incorrect labels:

They show me no proof of this and then add they cant locate the box because of the items being labelled wrong.

My question is I want the shipment returned to me ( to prove it was all correct) as it is my property, but it seems clear they have lost it how do I procced? has anyone else ever had this issue?

Any suggestions welcome

Thanks in advance