Are there BBs in the mag? Can you release them manually then you pull back the little switch next to the BB well? (
point it away from you, so you don't shoot BBs into your eye or something)
If BBs are inside and won't come out, then something is stuck (and on that note, don't use crapola cheapy BBs, they'll jam in your mags, they'll jam your in your barrel, they'll use up the last piece of toilet paper and not change the roll, they just aren't worth it). You can try knocking it free, or try unjamming it by sticking something down the BB well (like a small jewelers screwdriver. Again, be cautious so it doesn't fly out and hit you in the eye when it unjams.
If your mag empties okay, then the problem is somewhere on the gun and you'll have to do more checking to find out what's going wrong.