Adjust your stitch length to longer stitches that will allow for the thickness of the quilt. Then try it with a "scrap" quilt sandwich--three layers--top/batting/back to see if that helps.
I would also highly recommend purchasing a walking foot. I firmly believe that our quilting projects should be for the most part highly enjoyable....therefore doing everything we can to eliminate stress and frustration. Go get that walking foot---you will not be sorry!