Help! Machine (Sometimes) not Stitching/Skipping Stitch Sewing Machines Parts and Sales in Los Angeles

I really hope someone here can help me out, because I am baffled. I’m making pairs of wings out of heavy vinyl with satin stitched details. I’ve made 5 pairs no problem, and then all the sudden I start having issues with skipped stitches. I tried changing the needle (using leather needles), re-threading the machine, and adjusting tension. I even went out and bought a walking foot. Nothing worked. Actually it has started to get worse. At first there were a few skipped stitches, but now I’m getting whole areas that aren’t being stitched at all. And the bizarre thing is that it only seems to happen in certain areas. I’ve tested scraps and not had an issue, but then once I try to re-sew the messed up area, it still doesn’t work. Here’s a picture of affected area along with a scrap piece where it seems to be sewing fine for the most part, although you can see some skipped stitches there, too. A friend suggested that it might be a timing issue, but I’m not sure.