Not trying to insult your intelligence here but are you pushing the mag release all the way in?  You may have to remove the mag and look up inside the mag well while pushing the release to see if there isn't some flashing in there that could be catching on the mag and not letting it drop freely.  If that's the case all you need to do is remove the release and sand the extra material off, but do so carefully so as to not remove too much and thus screw it up.  
 
Is the mag showing any unusual wear?  Are you sure it's the mag release that's catching and not the insides of the mag well rubbing against the mag?  This is common and easily fixed by sanding the inside walls where the rubbing is taking place.