[LAU] rosegarden midi mixer has no effect

Hi all. rosegarden 1.7.3 sending midi data to either fluidsynth 1.0.9 or timidity++ 2.13.2 (tried both) using the fluidR 122501 soundfont when i play a midi track, and i change the volume levels in the midi mixer (eg turn the guitars down so i can hear drums better) this sometimes works temporarily (until a new section starts), and sometimes doesn't work at all. So at many points in time I see on the midi mixer that the volume levels of certain instruments are way down, yet i hear them loudly as usual. When all sliders are down to 0, the instruments will be muted for a while, but after a while, they become as loud as before again. what could be wrong? thanks Dieter