Some times not using the right size needle will cause that. If you have tightened the top tension to no avail I would try all of the following in succession, testing after each action:
As others have stated check to make sure the presser foot is down, probably the number one cause of the problem you describe for quilting machines that require it (my longarm does not)
completely rethread the machine
change the needle to a larger size
Loosen the bobbin tension