It's pretty frustrating not knowing what you don't know. I like the idea of approaching the band director privately and asking what it is she hears that you are doing wrong. Specifically, tempo, pitch, and , vibratto. I would try to not be defensive or make a case for why you are correct. Even if you don't agree. Ask questions if you don't understand but not with the attempt of justifying yourself. If the teacher is interested and you seem teachable then you might learn what it is you don't know. or you can look for reasons to justify your playing and not grow. Is it possible she likes the trombone player more than you ... maybe ....but it's more likely she enjoys good music.