Hey there, I was playing in my high range which just a few notes above the staff. I tried to go for a high c which is the note that sometimes comes out and sometimes doesn't. Only about 7 minutes into this, my lips wouldn't vibrate when I played high. The tone of the note would blast out for a moment but I coulnt sustain the note. I was using proper amount of breathe and air, I made sure my shoulders weren't all up and crazy. I made sure my aperature was open. No matter wwhat it felt like my lips just wouldn't sustain the notes this never really happened. I know endurance isn't forever but I was barely done with my warmup! I gave up before I destroyed my lips and just did a warm down and chilled. This has been going on for about 3 days of practice..