Do you mean that the thread on the underside of the stitching is wavy? if so, yes it sounds as if the bobbin thread is not under tension.
Take the bobbin out of the bobbin case and reload, make sure that the bobbin is in the right way and then hold the bobbin still and press the thread tail into the tension spring until you hear it click in. Usually the thread will come off the bobbin, through the tension then reverse direction in a "v" shape.
I think this is your bobbin case. So bobbin would go in and rotate anticlockwise, and thread would go into the tension spring south, then reverse direction north so rotary hook catches the thread.
Just make sure thread is fully into the tension spring, pull backwards on it and it should stay in the tension device if its in properly. If not, it will come out.
If you can't get a resolution then the tension spring may have failed, you can buy replacment bobbin cases from Brother, a local dealer or
I trust this helps you resolve the issue, often its just threading up and a bit of a break away for five mins, then fresh eyes, rethread carefully, checking the manual directions, and issue will resolve.