Registered iPad not listed

Hi,

I have 2 different Apple IDs.

On one ot these accounts, I registered all my products.

I got a new iPad and try to register it.

When I do it on the account on which all my other products are registered, I'm said it's already registered on another account.

When I switch to this other account, if I try to register my iPad, I'm said it is already registered (without mentionning on another account) but I don't find it in the list of my registered products (the list is empty).

So, where is it?

Thank you for your help