If the reminders didn't post to the group's page that's definitely a red flag. Either you haven't set up the reminders the way you think you have, or there's some kind of glitch.

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Gmail doesn't too often goof on spam detection, but just in case if you don't see the Spam folder in the list on the left (below Inbox, Sent Mail and Drafts) then drop the More menu. Gmail has an option to have the link to the Spam folder always visible, but I think the default is to tuck it away under More.

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Also with Gmail there's the tabbed Inbox. It is possible that the reminders were delivered to Social, Promotions, or one of the other configurable tabs.

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For rejections go to the Subscription page in the groups in question. Click on the Email Delivery History tab there and see if the reminders are listed as recent bounces.