Why are some tracks muting randomly?

Hey guys. I'm new to GarageBand for OSX so hopefully this will be something I'm just doing wrong, however I have an issue whilst trying to make a track,

I've set a certain loop and I'm trying to export it. However when I play it, only certain tracks play. The silent ones aren't muted and they're not showing as making any decibles. Yet some of them randomly start playing if I leave the track playing.

What determines whether they play or not or the point from which I play the track. However if I play it from the beginning they all play. The problem is, I don't always want to play the track from the beginning. And I'd like to export this one loop and they're silent on the export (as I was hoping that exporting would ignore the problem).

Tried googling the problem but I seem to be the only one who has this random silence problem.

Many thanks in advance.