ON SOME TRACKS THE VOLUME IS GONE

Hi everybody

I work with cubase pro 8.5 and on some tracks of my project the sound is gone…even if I restart the pc the problem is still there, in the inspector my sound card is select and the tracks are not muted >>> but the strange thing is,if I put on the left button of the mouse on the volume fader in the inspector of this tracks the sound is there and if I release the mouse botton the sound is gone…someone who knows how to solve this problem???
thanks!
Stefanie