Instructions
Check your grip. Glocks must be held firmly with a locked wrist to function. If your grip is weak or off center, readjust it and try again.
Unload the pistol and inspect the ammunition. A damaged bullet may be the culprit. Replace the bullet and try again. It is possible that the ammunition you're using is defective. If you suspect that's the case, change the ammunition.
Check the cleanliness of your weapon. Glocks are designed to be tough, but not neglected. If you're pistol is exceptionally dirty it may fail to operate. Disassemble and clean your pistol, then try again.
Replace the recoil spring (if the pistol still fails to operate). The failure to fire may be caused by a damaged or weak recoil spring. Use a spring made or endorsed by Glock.
Contact Glock customer service for instructions. At this point you have exhausted your options. If you're pistol is still failing to fire it requires repair by Glock.