As noted by many, the reminder-not-showing-up problem seems to often be related to a series, where the series has a reminder set, but the next occurrence does not have a reminder set (even though the user did not remove the reminder for the next occurrence).
Lately, I've been seeing a new manifestation: I will get a notification for the reminder for the next occurrence on both my Android phone and my PC, but before I have a chance to open the notification, it will automatically be dismissed from both my PC and
Android. Very annoying.
I've really got to move to a working calendar program.