Whether or not you see your fertile window is dependent on whether or not your status is "Trying to Conceive" or TTC. If you are actively using certain birth control methods, the app will also not show a fertile window. For example, if you're on the pill, Glow will not display your fertile window because the hormones in the pill are preventing you from ovulating and having a fertile window.
Also if your period is marked as late, the app also removes future fertile window predictions. When you confirm your period for that cycle, the app will add a fertile window prediction as well.