Know your enemy- 1. Excessive mouthpiece pressure One element which can limit your playing in the upper register is excessive mouthpiece pressure. The lip needs to continuously vibrate when playing and an excess of mouthpiece pressure can limit and even stop the lips vibration when playing high notes. One trick which will reduce mouthpiece pressure would be to change how you hold your horn. Check out these previously posted articles on the subject- How to Identify and Deal with Excessive Mouthpiece Pressure- Part 1 How to Identify and Deal with Excessive Mouthpiece Pressure- Part 2