Often puckers on the back of a quilt are the result of over tensioning the back. I was advised by the longarm teachers I have had to allow the back to be rather "soft" so that you can poke your finger up from the back and grab it from the top. I also stitch my sides with every roll of the quilt before using my side tensioners. I think I have had only two back puckers in over 100 quilts.
julia