I've been on the wrong side of this one...
I had a nice Jawa that was bowed and tried to flatten it and put it in a case. Both attempts actually caused the bubble to lift a little bit. Luckily it was okay (and I had it AFA graded with safety rails around the blister) but it wasn't meant to be flattened.
Best advice is to examine it carefully before attempting anything. If the seal is good, and the card can bend into a straight position without the bubble flexing then lay two big books on it. One book going across the top of the bubble, then another down the side or whatever angles help flatten best.
Then FORGET ABOUT THE MOC FOR A FEW WEEKS!!!!
Don't check it every five minutes, leave it be. Eventually it'll flatten out a decent amount and slide into a case. To avoid side damage, fold a piece of paper on the ends of the card, then slide into the case, then slide out the paper. Works like a charm