?I'm not sure what you mean by "the app". I assume you're talking about the calendar page of a Groups.io group. If you're talking about some other kind of calendar please be specific.?
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It sends them sometimes.
?You mean that sometimes you receive the reminders in your email? See my prior message about checking your email delivery.
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It did not post them to either of the affected groups.
?If the reminders do not appear in the Messages list of the group's web pages then they would not have been sent out by email either.? So that contradicts what you said about it sending them sometimes.
I think you'll need to be a lot more specific about what you tried, what you observed, and where you observed it before I'll be able to give you useful advice. A screen shot of the Reminders section of the event's edit page might be very informative. It ought to look something like this:
You might have more than one reminder for a given event, and would likely send one more than five minutes in advance, but that's just an example.