Simply disable Avaya add-in if you cannot see outlook reminders popups
MS Outlook 2016 was not showing my reminders popups since a few weeks, first I checked reminder settings in outlook:
File --> Options --> Advanced --> Reminders
Finally I found that disabling Avaya will solve the problem, here is the steps:
Goto: File --> Options --> Add Ins
Make sure "COM Add-ins" is seleted then click "Go"
Uncheck "Avaya Collaboration Services"
Click OK then restart outlook, all missed reminders will popup in outlook window.