Missing FBA Shipment

I have shipped a pallet over 2 weeks ago via Amazon shipping service which uses UPS and is still showing as shipped and collected but not in transit. Seller support have been less than helpful initially telling me I needed to contact UPS myself to find the pallet. After complaining that I am using Amazons shipping service and paying them directly for the service so it should be them chasing UPS I was then asked for the signed Bill of Lading and packing slip which I have duly provided only to be told the documents do not meet the requirements of acceptance?

All I want is for Amazon to find my missing pallet which I have proven has been collected by their partnered carrier. Can anyone advise an escalation process for this issue as we have now lost over a week of sales and lost stock worth 10s of thousands of pounds.