It sounds most like a tension issue to me. Play around with your tension on some scraps of the same fabric until you get a nice even stitch on both sides. QuiltedSunshine has a good suggestion that you use 2 different colors of thread and use a zig-zag stitch to check.
Also, check your pressure on your pressure foot. If you're sewing on thinner fabrics, the fabric may slide just a little and cause the stitch to be a little looser. Your foot may need a bit more pressure.