I have been playing euphonium for about 3 years now and am going into my junior year of high school and I have had this issue for a year or so. When playing I can only go up to a concert Bb4(or Bb 4 ledger lines up bass clef) consistently, every now and then I can play a C and D but they are never full and are very airy still. I feel like it may be a combination of things like too much pressure and tension, but I also think that there is an answer that is more obvious then I think. Thank you in advance for tips