Is this a new machine - or pre owned? The only time I had this issue with my Brother was when the embroidery unit needed servicing. Not all techs realize the unit as well as the machine need attention. Mine didn't.
Also, when embroidering on knitted or towelling fabric, you should use a washaway stabilizer on top as it gives a better finish.
Hoop stabilizer, fabric and washaway, so the fabric does not move.
If this does not work, and the machine is new, I would take it back to the place of purchase for testing. The shop should give you free lessons anyway to explain how it all works.
Good luck