Any one know why as I am sewing the full length zipper on my hammock I am getting thread bunching and balling? The hammock is very smooth material, the zipper is like webbing, I am sewing inside out, smooth material down, thread tension zero, stitch length exactly middle of dial. all other seams and stitch lines were great, only having trouble on zipper. its like it jams up and the material stops feeding on the dogs. I mean its sewing, but looks like crap. Any suggestions will be followed, I don't know squat.