Puckering can occur if you pull the stitches too tight or if you have thin fabric. An embroidery hoop with the fabric pulled tautly is your best option to keep your stitching flat. Conversely, if your stitches are too loose they can also sag and pucker so it is important to keep the stitches flat and even at all times. For thin fabrics, try putting some stabilizer underneath before you start stitching.