Compressor volume changes during usage

I have a very strange issue: After playing with along with a song on an audio track using a compressor on the voice microphone track (and a e-guitar on another track) with monitoring the volume of the compressor changes.

After a while it becomes less volume?! Turning the compressor on and of with the insert effect switch on the left fixes it?

Any hints?!

Thanks a lot, Detlef