How do I get 2 tracks to play at the same time

Question

I don't understand what's going on or what to do about it.
I have a track Kits that I can add any (apparently) kind of sound to, but when I drag a bass to a new track it's gray and won't play, or else it will play but not the Kits track. And why is it gray, I thought gray would mean 'grayed out', won't play but no.
Then, somehow a yellow bar has appeared over the first 5 bars and those bars will loop...unless I take the playhead past them--while the music is playing--and subsequent bars will play. But if, when the music is stopped, I set the playhead past those first 5 bars and press play the music starts from the first bar.
You'd think the yellow bar would indicate some kind of rendering, but not that I can tell.
I'm on a fairly new MB Pro. Seems like my old one with the older version of GB didn't have these hangups. I look through the drop-downs or fiddle with some things, but don't see anything to correct the problems.
Any ideas?
Thanks.

Answer 1

Thanks for the help folks, sorry I've been away.
I'd love help with the problem of why I only one track seems to accept clips to play. When I drag them to other tracks or create a track by dragging the clips are grayed out and won't play, but if I drag them to the one track where they're blue they play.