Help with linking seller account to registered brand

We have a brand registered with Amazon Brand Registry - but every time we try to link the Brand we have registered to our selling account on Amazon, we keep getting this message - "We are unable to proceed with the request due to account eligibility reasons" We have opened multiple help cases with Amazon Brand Registry support but despite going back and forth multiple times nobody can give us any information on how we resolve it and the reply from Amazon brand support is just the same generic reply every time. Our account health is good in all Amazon countries we sell in. Can anybody advise how we solve this issue?