This is a mellophone question but the answer may apply to trumpets too (and I didn't find a mello-master forum).
I have a Bach 1105 mellophone pitched in F. I'm having a terrible time with the F at the top of the staff. I can hit the note below and above in the overtone series but the F is killing me. I thought it might be either the MPC (which changed) or me (I'm a woodwind player). So I asked my son who is a French Horn and mellophone player. He had the same problem. I tried hitting that note on the King 1121 he plays at school with my MPC. No problems at all with that note (beyond the bit about sounding like a woodwind player who shouldn't be allowed near a brass instrument). The other piece of the puzzle is I had loaned the instrument to someone for a few months. I don't recall issues but I couldn't honestly tell you if I'd exercised that note either (it doesn't get played so much). So that is the background. My question is if there is any sort of mechanical issue which would tend to cause an issue with that particular note? I didn't notice issues with other notes which have valve 1 pressed. Nothing jumped out at me visually on the outside of the instrument. It is certainly possible that this issue existed all along on this instrument as it really has not been played much by anyone in my house.
Thanks for humoring a brass challenged person.