This happened to me last Spring, the bobbin had a couple of burrs on it, the repair folks discovered there were two, they fixed it, and when I picked it up, the reverse button didn't work, however it was working when I took it in, they
found the motherboard needed to be replaced, and the reverse button had its own board....was in the shop for 6 months, and thank goodness they gave me a loaner to use. My machine is a Bernina 720 and not that old and was interesting to find out those things went wrong so soon.