Several things to try, I have this set up and after six months I'm still trying to get some issues resolved.  First make sure the machine is treaded correctly, the Juki doesn't thread like most machines, make sure you have the bobbin going the correct way.  After the machine then it has to be in the frame which means that the fabric is most likely to tight, this is a common problem, if you are bending the bars then it is too tight.  Also, look at how the quilt is in relationship to the frame, is it level?  As you quilt and roll, you will need to adjust the frame up, the quilted part should not drag on the machine and the to be quilted part should not be below the machine.  It is a critical that the frame and the machine be even with each other, if not the machine will try to pull and won't sew correctly.  Also, another tip, after each quilt change your needle, the reason is that free motion is technically bending the needle.  Hope all this helps.