In a drop-in bobbin, sometimes the "bump" on the plastic bobbin case (on the right / front) gets a scratch on it. Then the thread gets caught on that scratched plastic. You can fix it by using a very fine sandpaper (about 400-500 grit). You only need to brush it with sandpaper about 3 or 4 times -- be very gentle. I see the damaged plastic bobbin cases a lot.
If that's not it, it could be that the bobbin case retainer needs to be adjusted. That's a simple fix that your sewing machine tech can do.