Find my Friend not always notifying. After resetting phone have to force close the app or if it was closed you have to start it in order for notifications to start being sent. This is only since 9.3 iOS updates
iPhone 6 Plus, iOS 9.3.2
Posted on Jun 10, 2016 8:53 AM
Q: Find my Friend not always notifying
Sounds like a bug, most likely you will have to wait for the next update for it to be fixed.
Have you tried force restarting or resetting your phone? or checking under settings>notifications to make sure everything looks ok?