ERROR - You are using codes that do not match

I have been selling on amazon for many years. We manufacture our own products which are unique to us and so we have to have a barcode which we get from GS1. Last weekend I got an error code saying that I was using an ean that didn’t match the product I am trying to list. On the following day I listed an item with 23 variations without any problem but since then everytime I try to list a new product I am receiving the same error code. I have been in contact with amazon all week but getting nowhere. Amazon requested a copy of our GS1 certificate which I have send to them but the problem is still persisting. Can anyone advise what I need to do? TIA