You mentioned in an earlier post that you weren’t sure but you may have slowed your arm speed when you threw a changeup. Big mistake. You have to throw all your pitches, and I mean ALL of them, with the same arm motion and the same arm speed as your fast ball. The secret of the changeup is in the grip and the way you hold the ball.
I remember way back when, in my playing days, we played a lot of games against a team that had a pitcher with perhaps the prettiest slow curve I had ever seen. His problem was that he was forever telegraphing it. He had a funny way of wiggling his elbow when he was going to throw it, and he also slowed his arm speed—a no-no in any pitcher’s book. You may be sure we drove him off the mound in the third inning every time!
As for the problem you’ve been having with the knuckleball—don’t scrap it, for Pete’s sake! Take a look at the way you’ve been throwing it; you’re not supposed to have any arm problems with that pitch. It might be something you’ve been doing with the grip that puts unnecessary strain on your arm; try holding it a bit looser, and the heck with what everyone else says, throw it like the fast ball. Also, you might try experimenting with the knuckle-curve which can be an absolutely devastating pitch—just find a good knuckleball grip and throw the curve. It will drop sharply, like a glass crashing to the floor, and the batters will start screaming because they can’t hit it. (I used a sharp karate-chop wrist snap for both my regular curve and the knuckle-curve, with excellent results.)