Was having a similar problem recently. This is probably not what is happening to you, but it is at least worth mentioning.
The bottom of the presser foot on my machine was gummed up with glue from earlier sewing. It adhered to the top layer of fabric, which, after the first couple of stitches, held both layers of fabric so that they bunched up behind the presser foot and stopped forward forward motion. It took some detective work to figure that out, and then some elbow grease to clean it up. I had to do clean it several times, but finally was able to sew again. Now I am much more careful when using gluey materials while sewing!