What size machine needle do you use? I have tried using a size 14 and my zig zag stitch is messing up every few stitches.It is failing to grab the thread beneath, on the right side of the zig-zag. I have tried it on 2 sewing machines. I have all of my settings correct. I am thinking maybe I need a sharper point needle, like a jeans needle 16 or 18?