If you mean you had your mh number on, you are going to struggle a lot. You’d need mh to first cancel the (non) credit to them, then send ba confirmation they have whilst you try retro credit with ba. At best it will take a lot of time and energy insisting, but I think there is >50% chance it most simply won’t work at all as mh don’t care and ba will see your mh number on the bp and refuse credit.

if I misunderstood and you never had another number then do write then call if need be. Took me >3 months to get a simple aa flight credited and multiple mails and calls and I’m ggl and aa are usually cooperative.