make sure the 'latch' on the bottom of the gun, does NOT have any 'gunk/crud' stuck in or around the front 'notch', and also that you did NOT chip or break off this notch and then make sure the latch is working properly and the spring did not hang up or come loose or break, has to have good tension to maintain pressure on the bottom ,back edge of the magazine, as well as the back edge of the mags themselves are NOT 'rounded off" or chipped ( this happens if dropped, and they land/hit on the bottom edge) yes, a few different things to look at and for.............
also have seen it when one puts the receiver (upper) back on the gun, and its 'tight" if you 'canted' the receiver tube off to one side or the other, may be catching the lips of the magazine, not allowing it to "seat" ( lock in) all the way................
if its doing it with all 3 mags,does not sound like its the mags themselves...
as for the aftermarket grips, this may be the holes are too deep and the screw is going into the frame too far, hanging up the sides of the magazine.