Double clicking a bar doesn't start song any more, except for the first bar.
The chord just sounds once instead. I have to play all songs from the beginning every time now. Have I accidentally 'unfeatured' something? How can I mend this?

Hmmm... Not at my music computer right now, but i do know that I performed this action earlier this morning, so I know it works.

Probably not your problem, but make sure you are double clicking on the white part of the bar, not the bar number, as that is used to change part markers.