I hade to factory reset my kindle paperwhite (6) to reset my parental controls password. Now I can't stay registered! It registers the account, then kicks me out again a few seconds later. It's hugely frustrating!!
That's something Amazon customer service is going to have to help you with.
Go to Help>>Need More Help>>Contact Us and select the phone or chat option for best results.
I'm being extremely clever up here and there's no one standing around looking impressed. What's the point in having you all? - Dr Who
I have this very same issue. Reset the wife’s kindle to move to my account to get the unlimited books but won’t stay registered.
The same answer applies to you.
Contact Amazon customer service.