Phone not backed up to iCloud

I periodically get the message on my iPhone 6 that it hasn't been backed up to iCloud. This message says it hasn't backed up in 8 weeks, and that in order for that to happen, the phone needs to be plugged in, locked and connected to Wifi.

For most of the past 8 weeks, this phone has been plugged in, locked and connected to Wifi. Why is it not backed up to iCloud?