If it's the original bobbin case, under the spring may be packed with lint. You can carefully! remove the screws, take that spring off and wipe both it and the case down, then put everything back together again.
If you do this, do it over a white towel-if you drop a screw it won't bounce, and will be easy to spot. If you've got two screws, make sure you know which one came out of which hole-they're often different lengths. An eyeglasses screwdriver is the perfect size for this. After you've got it back together again, you're still going to have to readjust the tension. I find it easiest to go backwards-that is, screw it all the way in, try it, and then back off 1/4 turn at a time until you get it where you want it. Once you've got it there, you can mark the case with a fine tip permanent marker-I usually start the line in the screw slot so I can line it up again if I need to.