This problem has many names: tangling, nesting, clotting, boogering-up....
Whatever you call it, it can turn a good sewing day into a nightmare.
It doesn't have to be this way! Here's what's going on, and how to fix it:
Much like the Three Stitch Problem, Nesting is the symptom of an upper-thread problem.
This may sound silly, since all the trouble's in the bobbin case, but it's all about the Upper Tension.
The Upper Tension is controlled by a dial.
When threading the machine, the upper thread loops around the tension control before heading up to the Take-Up Finger.
If you loop around the Upper Tension while the presser foot is up, the thread slides nicely between the tension disks as intended.
Just give that upper thread a simple little tug to make sure it's safely in the tension disks.
Some Upper Tensions are hidden in the machine, so you won't be able to see the tension disks, but gentle tugging will still work.