Some ideas...bobbin tension should be adjusted first...have a look at Jamie Wallen's video which I'll attach.
Is your needle the right size for the thread? 40 weight will take a 110/18 or even a 120/19.
Have you dropped your bobbin holder lately? If it becomes out of true, it won't work correctly.
Are all your threads in the the thread path correctly? Your thread isn't getting wrapped around the pin at the spool?
Change your needle...it may have a burr on it.
Check your connections underneath that attach your machine to the track system. I spent two days trying to fix a thread break only to find out that one of my connections was loose that connected the machine to the roller system underneath. (Can't think of the technical terms.)
Tension video
Watson