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?