I have the same problem with the bass and drum track, but only when I just want to export the chorus of my song.
When I export the entire length of the song it works just fine and has bass and drum included.
It's puzzling and I don't complain about free software.
Since I only need to export the chorus so a singer knows the melody and can record her version for me, I just export the entire thing and cut out the chorus in another software.