as a loooooong time Glock shootah....I can tell you 3 things that cause this:
1) As
@Bender said....on alot of trigger shoes there is a "mold line" on the front. You can take some fine grit sand paper and remove this for more comfort.
2) The trigger safety tab (the piece that engages the frame when the trigger is released and is pressed in when you place your finger on the trigger)...sometimes is not flush with the trigger pad and sits slightly above the trigger face and not flush. So when you place your finger on the trigger, the safety tab not being flush will cause an irritation. Take the trigger bar out of the gun, hold the safety tab engaged from the rear and any little bit that is protruding above the face of the trigger shoe, just trim it with a razor blade (it's usually minimal) until it is flush with the trigger shoe when depressed.
3) Do you have a "serrated" trigger shoe or a "smooth" trigger shoe? I find the "serrated" trigger shoes cause me issues with extended range sessions (300+ rounds). I put smooth trigger shoes on all my Glocks.