u/Intrepid-Pen-2556

I play a Thomann superior, nice sound good quality. But always after playing for an hour of more, it feels like I am blowing against a wall. All valves are clear, no water in the tubes.
I cant figure it out what is wrong. But I can’t get a good airflow. Idea what i’m doing wrong of where to look at on the euphonium?

For the record, I was playing bes bass for 30 of so years untill january 2025.