I've been doing some buzzing exercises on my mouthpiece where I do octave slides from Bb on the second from bottom line of the bass clef, moving up in semitones. My highest is the octave slide starting on the first D above the bass clef. For some reason, I've been stuck on that note (the D the octave above the first D above the bass clef) for a couple months now. For some reason I feel as though I physically cannot make my aperture any smaller; my embouchure any tighter. When I play into my trombone I can play the F a minor third above that (albeit not very cleanly).
In summary, my question is this: Is there anything I can do to be able to buzz higher (and in turn increase range)? Why is my range smaller on my mouthpiece as opposed to on my trombone? And lastly, will expanding my buzzing range strengthen my higher trombone range?
Thanks in advance.