Problems with lower register.

I`ve been playing euphonium for about 3 years now, and took trumpet up about 2 years ago. I`m grade 4 on trumpet and grade 3 on euphonium and taking grade 5 on both this year. Previously, the playing of the two instruments has had no effect on each other. But recently, my trumpet playing has deteriated a lot, most notably i seem to lst the ability to slur to notes lower than middle C. Any suggestions people?

I know what is happening, I have recently played the euphonium, and, I have been playing trumpet for 4 years. One of the reasons could be that, when you are playing euphonium, your emboucher(sp?) is much loser than on the trupmet, and also since the mouthpiece is much smaller on a trumpet, it means that you can`t hold your lips tight enough. So my suggestion, is to practice your euphonium for a little while, and then, take your trumpet and do the lip slurs, it will feel funny at first, but after a while you will get it back, and also playing euphonium extends your trumpet range much higher. I am currently able to hit a G above high C, and, the euphonium player in my school`s top band plays trumpet in jazz band, and can hit a super C.