I have a student that got braces off about 3 weeks ago.
Before the braces came off he was getting a nice sound but not a lot of range. He also plays to one side. I figured that the braces were causing some pain and he found a comfortable spot.
When the braces came off, he had a lot of trouble. We talked about centering the mouthpiece because he was starting a new thing anyway but I didn't push it. I had him center it and play so he could feel it and get the idea and then left it alone. The next lesson it was back where it was with the braces.
Yesterday he had a lesson. I had assigned Clarke. The second study from (I think) low C down to G. He played beautifly. Nice sound!
Next he had Arban. A song that didn't go higher than G over the staff.
First note great and then, fluf, fluf, fluf. We worked on sound for a long time. Bell up, full breath. When he does this and plays MF or F he is OK.
Anything softer and it's like the lips stop buzzing. Back to Clarke and he is OK.