The problem is with mostly recent production, only 40cal Glocks. It effects the Model 22, 23, and 35 when mounted with a light and sometimes without a light. The issue is that the 40 cal round is powerful and causes alot of slide velocity. The GLOCk frame needs to flex to ensure proper feeding of the next round. With the light mounted on the frame, the flex is changed or minimized. This causes the weapon to malfunction, usually the next round nose dives and doesn't feed.
The fix that GLOCK is currently recommending is that you get a generation 8 mag follower, an 11 coil magazine spring. Also, keep on top of replacing your recoil spring assembly (its a very cheap part). The recoil spring assembly should be replaced about every 3-5K rounds. This is the way that GLOCK recommends minimizing problems with feeding when using a weapon mounted light. Keep in mind, however, that if you call GLOCK customer service they will likely blame the light and tell you to use their brand light (which sucks). however, these problems have also appeared in weapons carrying the GLOCK light.
Topeka (iirc) KS PD and also Indiana State Police all got rid of all of theri GLOCK 40's because of issues in their reliability. The ISP guns were not even using lights. The TPD guns were some with and some without lights.
My best suggestion, carry a GLOCK 9mm if you can. With modern ammo there isn't that much difference between good quality 9mm defensive ammo and teh same in 40, however, GLOCK 9mms are unbelievably reliable. 40's, not so much. I recall reading something that said this issue is only with relatively recent (since the frame redesign in 12/05) guns only but rumor is that the newest ones have the appropriate follower and mag spring.
I concur..get yourself a good glock 19 or a 26 and get some good personal defense 9mm loading that the model likes and carry that.