Take the B board out of the shell and connect just the bare PCB to the A board, push it down SNUG. If you still have the issue then it is almost assuredly a rom issue.
Thanks for the replys so far
I did try the "naked" B approach, I did it with 2 diferent A boards.
but I didn't really take a good look at the connector of the B board, I'll just use some DeoxIT on it and see what happens.