You have to make sure that the thread is long enough when you start sewing and you have to make sure the needle has gone all the way up and then down a little when you want to stop. Otherwise the bobbin thread and the spool thread get tied in a knot underneath.
So when you stop sewing, turn the wheel so your needle goes up and then down a little and then pull your garment out.
When you start to sew make sure your thread is atleast 5 inches long trailing behind the machine.