If you are confident that you are not moving the sandwich too fast for the sewing speed, then it must be the tension or thread. Put 2 different coloured threads in the top and bobbin and sew straight seams to check if you have a balanced stitch. If that looks good do a zig zag stitch and check for a balance stitch. If your stitch looks good then it should work for FMQ. You can try different threads and see if it makes any difference.