I have trouble getting enough air in. What do I do wrong?
Both feet on the ground. Sit on the front end of the chair. Do not use the back rest. When inhaling the entire torso expands, including the back. Leaning your back against the back rest of the chair will make expansion harder and you will have to use much more energy getting the air in than necessary.
Sit with a straight back allowing the chest to expand freely. Stand up with your heels, back and head against a wall. Breath deeply. This is a good, open posture. Try to find this also when sitting.
While you grow taller the horn does not. If you started with a straight back with the horn in your knee, you must lift the euphonium accordingly when growing to maintain the straight back. If you are lazy and keep on resting the euphonium in your knee as you grow taller, you will end up with the shape of a banana! Restricting the expansion in all directions.
Short tuba players might want to sit on a pillow while resting the tuba on the chair.
Long tuba/euphonium player can use a pillow in their lap to put their horn on.