One common issue when stenciling cookies is the stencil shifting or moving while you're applying the icing. To prevent this, make sure to secure the stencil firmly in place before you begin. You can use a food-safe tape, such as masking tape or painter's tape, to hold the stencil in place on the cookie's surface. Alternatively, you can try using a stencil holder or hold the stencil down with your fingertips. Whichever method you choose, ensure that the stencil is stable and won't budge while you work.