Never had the pistol in question, but does it do it fully loaded and seated in the pistol? Sounds odd that the mags would loosen up like that in the pistol, especially w/ a low round count? Generally mags would do the opposite such as bulging a bit and not drop free fully loaded. Does that particular firearm have a polymer magazine release? Are the magazine cutouts overly worn? Only other explanation that I can think of is a wearing magazine well, but I've never heard of that one. I'd likewise want to say that either the magazines are out of spec, or the magazine well, but you've implied they were tight fitting once.