Actually, why did you think you had 10 in the back? When you shift, is it clicking for 10 or 9 gears? I'm curious only because if you don't match the number from the front to the cassette, it won't fall correctly no matter how you adjust it. It will jump between gears because it can't quite line up. When tightened, it will work for a short time and then be out of whack again almost immediately. We had a bike shop replace Kit's rear cassette and they didn't count the # of gears before the change. Guess what? They had it wrong and couldn't get it set. They put on the correct cassette and no problems after that.
Check your brifters (or whatever set up you have for changing gears) and see how many clicks you get compared to the cassette. Good luck.