Q: Iphone 6 calendar stopped syncing with Outlook

My iPhone 6 calendar sync's with Outlook via the iCloud, but has stopped working properly.  When I enter an appointment on my iPhone 6, it appears in my iCloud calendar, but not in my Outlook calendar.  However when I enter an appointment in Outlook, it appears in my iCloud calendar and on my iPhone 6. I'm on iOS 9.3.1 and Outlook 2016.  Can anyone help me with this?