What kind of machine is it? Are you sure you have the upper thread in the thread guides. It sounds like the thread is not staying in the upper tension

If machine has horizontal spool, be sure spool cap is larger than spool and thread dispenses from spool by falling forward (flip spool on pin to correct).

Be sure top thread tension is not too loose.

If still problems, thoroughly clean bobbin hook/race area per machine instruction.

Set top tension at four. If the ball of thread remains on bottom then loosen the bobbin tension. Do this a little at a time until the stitch is good.