I recently found it quite difficult to begin serging a slippery knit because the fabric kept getting caught up in the feed dogs. I solved this by placing a thin sheet of paper under the fabric. I used some thin paper designed to stabilize embroidery stitches but you can probably use regular paper or tissue paper. The paper tears off easily and if there's a little left in with the stitches, it will probably come out in the wash. I only used the paper for the first inch or so because I did find that the paper caused a little puckering. In my situation I was using a coverstitch - for some reason the fabric had no problems with overlock but when it came to the coverstitch I just couldn't get it started. The paper solved the problem.