Try this:

Remove the top thread completely from the machine.
ALWAYS RAISE the presser foot and rethread from the beginning.

Now test your machine.

Be careful what TYPE of bobbin you use. Some are flat and some are curved. Sometimes the plastic ones are even different sizes! See if you can determine what type CAME with the machine and only use that type. JLP

Possibly your thread tension is too loose--or your machine may need cleaning in the bobbin area.