What are you trying to sew?
Is this only happening when trying to do FMQ and if so make sure that you do not have weight of the quilt causing any drag as this can cause birdnests. Trust me I know. Make sure that you have your quilt well supported and fairly even with the sewing machine. Even try turning the machine on its side and build up an area to support the weight.
Also are you using the same thread in the bobbin as the top - this can also cause a problem.
I hope you sort the problem out soon.