Okay I have just had a very interesting session of practice.
First the background. I set my mouthpiece very low, which is probably why I like the asymetric trumpet piece. I think this is because as a child I was given a very small deep cornet mouthpiece and as I grew I never changed the set position on my top lip. After a long layoff I set a newer larger mouthpiece on the top lip where I used to and was away
So early this year I went to a teacher who decided my set was far too low and got me to try to switch to a fifty fifty set. Oh boy have I strugled all year with sort of blind spot between Ab above the stave and B then it opened back up and I was able to get up to G above the stave. Now the asymetric has give me the clue, I returned today to a deliberately low set and bang everything fits again. I have parted company with the teacher and am with someone else who is encouraging me to paly as naturally as possible.
Just sharing guys