This sometimes plagues me too. Some fabrics are worse than others, and the grain on which they are cut can be a slip factor. When I see a slip pattern occurring, I take a little extra time to starch my pieces together. It helps to have a mini iron and press board next to the machine so I don't have to keep jumping up to tack the pieces together. It makes a big difference with those uneven edges though. Well worth the extra time for me.