I thought I had a real lemon in my G19, in that it'd feed FMJ flawlessly, but fail to feed on every JHP I tried. Was a 3rd gen gun, with 9mm-1 followers. I called Glock and they sent me a couple 9mm-3 followers, made no difference. I ended up sending the gun back, along with all mags. They replaced the mag followers with 9mm-3s, replaced the mag springs, polished the feedramp, and replaced the slide stop. Guess what? Still fail to feeds on JHPs...$45 wasted. I fired at least $500 of JHP ammo trying to get the gun to work.
I was able to get ahold of the new production (post ban) mags with the -5 follower, and all has been well.
I suspect that the old mags were out of spec in some subtle way; the moral is to use new production mags in a Gen-3 gun. Sucks that I laid out $300 for 4 preban 15-rd mags and they refuse to work properly.
I would have liked them to replace the mags with new production ones that would work, but alas they wouldn't. So I have mixed feelings on the idea of buying any more Glocks...the one I have is now reliable, but the whole incident left a sour taste in my mouth as it were.