To raise your groups, switch to a heavier bullet, not a lighter one. The heavier bullet will exit the gun later in the recoil cycle, so the gun's muzzle will be higher when it emerges.
But first make sure that it's not you. Jerking its trigger makes a pistol shoot low, and the Glock "Safe Action" makes trigger jerking all too easy to do.
Grasp your pistol very firmly and lock your forearm joints. Try to concentrate upon keeping your sights aligned carefully on your target, while you slowly and gently press (not "pull") the trigger straight back. The gun should surprise you, when it goes off.
Now, where did that shot go? Fire at least two more (and four more is better) in exactly the same way, to make sure that it wasn't a fluke.