Q: Notifications clearing from lock screen

My problem is that notifications are periodically not displaying on my phone, it only seems to happen when I leave my phone charging overnight and doesn't happen all the time. At first it was for iMessage, I would receive a load of group chat messages which when I got up on a morning hadn't displayed as a notification but when I unlocked my phone on a morning they would be there, all unread with the number of messages on the icon.

Ive noticed this a couple of times now and last night at about 1.30am I woke up, decided to check the time and noticed my phone had absolutely no notifications at all?! When I checked my phone I had iMessage and when I slid down for my notification panel there were 4 or 5 apps that had sent me notifications. Why is this happening? I have an iPad and this is zipped away in a bag in a drawer. I've checked notification settings and they're fine?

I have an iPhone 6 running iOS 9.3.1.