I had one stretch of time where every time I finished piecing a row the thread broke. I think there was something screwy with the way I wound that particular bobbin - so I unwound several yards of thread and tried again - still broke. Inserted a different bobbin and everything went smooth as silk again.
I've also found that if I don't get everything firmly in place, when I reload the bobbin, screwy things happen. My machine is still fairly new to me.
Hopefully it is something simple.
Cheers, K