I spray the wrinkles well (with water, starch, whatever I'm using), then roll up the fabric and put it in a plastic bag for 30 minutes to an hour. This lets the sprayed droplets penetrate the fabric evenly on its own. When I take the fabric out of the bag and press it, out come the stubborn wrinkles! If you aren't going to be able to get back to it within the hour, you can store it in the freezer (bag and all) until you are ready.
Are you sure it's still a wrinkle and not a faded mark where the wrinkle used to be?