Are you storing it in like a really cold place? During a football game once, my trumpet valves literally froze (within like 30 seconds), and there was just a light snow out. Also, sometimes the third valve slide (the others somewaht less) get tiny tiny dents in them that effect the valves - even if you can't see them... maybe take it to a repair shop to just make sure. Another thiing...have you "given your trumpet a bath" lately? You're supposed to take out all the slides and valves and soak just the trumpet's "body" in water...run a snake cleaner through it to get out any buildup. There could be some in the valve slot, or in the valve. And remember --- valve oil! Good luck!

I'm going off of what definition said... replace screwdriver with wood dowl and you're all good. NEVER use a harder metal punch on softer metal. Alwase try to use something softer. ;-)