Shaving the bottom of the tab usually fixes the problem. But I've had a couple of boards where that wasn't enough. I was afraid of taking too much off the tab and weakening it. So I added some material to the bottom of the fin and that took care of it. I mixed some milled fibers into the resin for strength. I've had this problem with a couple of Chinook boxes from the 90's. Almost every fin I tried in the Chinook box was loose. You can see the thin line along the base in this pic where new material was added.