If you're tracking a birth control method with Spot On (pill, patch, ring, shot, implant, or IUD), you will never see a fertile window, as these methods prevent ovulation (and therefore fertility) while you're using them. However, if you are a pill, patch, or ring user and get off schedule, you will see a fertile window (if you are opted into pregnancy advice).
If you're not tracking a method in Spot On, your cycle may not be consistent enough for the app to predict your fertility. Ovulation predictions based on periods alone are not reliable unless you have very regular cycles.The app only predicts fertility in a tight window of regularity:
Regular Period (for delivering period predictions) = every 21-37 days
Regular Period (for delivering fertility window) = every 26-32 days