There can be various reasons for notifications not arriving on time or not at all.
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Check that in More > Settings > Notifications notifications are turned on, and they are enabled for the show you want to receive notifications for. Also check the other notification settings.
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Check that new episodes have a release date, otherwise SeriesGuide won’t know when to notify about them. Make sure that your device is configured to sync and update your shows: https://seriesgui.de/help#syncfail
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Power optimization settings of your device may delay SeriesGuide from running, delaying your notifications. Check the link to Battery Settings in More > Settings > Notifications and try to turn off battery optimizations for SeriesGuide.
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There may be additional aggressive power saving settings depending on your device that stop SeriesGuide from running in the background and dropping scheduled notifications. See https://dontkillmyapp.com/ for some pointers.