Drum Pattern Fades When Playing.

[Hobson]

I've got a combi set up with seven different pianos and a drum track. This way I can quickly switch between pianos by soloing one of them and the drum track.

Problem is, sometimes, without touching any controller, the drums get quieter while I play the selected piano, sometimes it almost fades completely away. When I stop playing the drums come back full strength.

Any ideas how to keep the drums from fading away?

Thanks.

Hobson