TubeNet

Mandrake's right about the lungs,especially if you smoke like I did for 45 years.The trick is to make'em think you still have some lungs.Folks get real crafty late in life and that enormous amount of experience can make you sound good in spite of any shortcomings.
That old horn could be leaking or partially blocked and that could cause it to not slot well.Last time my old sousie sounded that bad,I spun it a few times and a dead bat fell out.Lotsa critters down South.
It sounds like you're still having fun with your music.
Tubatooter1940

If all else fails, you might try some mouthpiece buzzing. I don't know if it will help but it is worth a shot.

Sousie