Hi & problem

[blitzbob]

Hi all
I'm bob from Ramsgate in the u.k. I've been playing trumpit and cornet  with a number of bands over the years. got a problem I've never come across. pick up a trumpit cheep and its fine til you use 1st & 3rd and then instead of a clear not it sounds more like a cow. had it apart and cant see any thing.
any ideas?

thanks
bob