I have experienced the same problems. Left the mags loaded with 10 rounds for 3 months, cut the spring, tried different springs, all the suggested above, the only thing that changed was that the 1st few rounds were easier to load. The mags will hold 10 rounds, the problem is that the last round has so much presure on it that it causes drag between the top round and the bottom of the slide. This can cause a malfunction and make it hard to seat the mag (you got to slap her on the ass real hard ). Flexmoney has got it right, the G21 restricted mags are the worst, my G17 restricted mags work fine. I too would like to know if any one has been able to get the G21 10 round magazine to function correctly. I have a gen 2 mag that I am will to grind on if anyone has a suggestion.
TIA
G45