maybe your ambusher (sp?) has to be changed.
Hehe.. close pianistimo, it's embouchure. It's pronounced almost like "ambusher," "ombusher" is the pronounciation.
Now, about yudei's problem:
What brass instrument do you play? I hear people refer to their brass instrument as "horn" all the time; do you actually mean French horn, or some other brass instrument? Where do you go sharp or flat? Only on certain notes, or it is just randomly? Are all of the slides pulled out or pushed in the correct length? That greatly affects tuning (because that's how you tune a brass instrument

). When you go sharp or flat, are you changing anything on how you play? Embouchure, breath support, the vibration of your lips?
The more info you can post the better. Or you could just ask you teacher or band director.
