Amazon Warehouse Refused Delivery

I've made perhaps 20-25 shipments so far this year, and Amazon have refused delivery of around 40% of them and returned them to me.

I have had shipments returned from Paris, Florida, California, Dubai, Saudi Arabia and Italy.

All of my shipments are identical, I am shipping the same SKU in the same volume in the same box. I really don't understand what I am doing wrong and why the problem is so hit and miss.

Has anyone every had anything similar? Curiously all of this shipments were made via Royal Mail.

The warehouse are not even opening the box, and I believe I am attaching all of the necessary labels and information. The shipment is getting all the way to the warehouse and then coming all the way back. Some of my shipments will have done 20,000km round trip.