Before you start messing with the mag catch spring you should think about these questions:
Do you have an extended release on your G21? If so, consider going back to the standard part.
Also, are you sure you are getting the mag fully latched in place? Especially when inserting a full mag on a closed slide, it can be difficult to latch the mag fully into the well.
If you decide to replace the spring, be very careful during the removal/installation. It needs to come STRAIGHT up out and go STRAIGHT back in. If you pull it out or push it back in at an angle, you can break the plastic around the hole it goes into--there's very little frame material where the hole is. If you ruin the hole, you have ruined the frame and that will cost you a pretty penny.