Morris pointed out fantastic things. However, for me it's all about playing chromatic scales note by note, at different tempos. It could be probably the lack of your muscle power. Try the Pencil Method, where you put a pencil in your mouth, and hold it there until you feel your muscles begin to work. Don't overdo it though, or you can do the squeaking exercise. The S.E. is you just try to squeak out high notes. It really develops your chops. Have fun!