At the shop, I see this all the time with drop-in plastic bobbin baskets. They just don't handle thick fabric. It's probably just fine while sewing two layers of cotton fabric. Right?
Check the bobbin basket for damage. You can polish any needle holes or scratches with a very fine sandpaper, about 400 - 600 grit. It only takes a few strokes, so be gentle.
The other thing that happens is the spring in the front can get damaged. Sometimes it needs to be bent back into shape or replaced.
One other thing that happens is the "finger" that holds the bobbin basket position gets bent upward. Most can be pushed back down with your finger. The metal is pretty soft. Some fingers are in the needle plate, and can also be adjusted.
There needs to be a small space between the bump on the front of the bobbin basket and the spring in the front, for the thread to pass through, so don't tighten the space too tight.