Problems with playback

I'm composing a trio. Flute, acoustic guitar and piano. For some good reason the bass of the guitar is quiet, while the dynamics is mf. G bass string for instance is very quiet, others a fine. What could be a problem?
The other thing, when I choose a certain measure to play, only the flute plays, others are get muted, as shown in the mixer. I tried pressing unmute, but it wouldn't work.
And the guitar sometimes bangs, instead of playing softly.