Using Classic Outlook on Windows for personal email, calendar, and contact management
Hi tw33k
Welcome to post in the community.
From your description, you can try to use quick repair tool to check if it is workable. Besides, please verify the settings about Reminders is enable(In Outlook client File> Option> Advanced).
Furthermore, you can run Outlook in safe mode to check if reminders can be always working. It is useful to verify whether the issue associated to add-ins on your computer.
If the issue persists, to provide further assistance, we'd like to collect the screenshot about Outlook client version to further check. You can find the version by the steps below (File>Office Account).
If you have anything unclear, please feel free to let us know.
Regards,
Joey