iPhone connected to charger and shows charging but not charging battery
It happened to me once. Your charger is an Apple charger? When I was using a fake charger it was a really problem for me, so my way to fix it was rebooting via LOCK + HOME buttons and then check if the iPhone was charging. It could be just a bug.

If it's still not charging, change your charger because it can be broke somewhere or getting voltage changes. Apple Support have all the graphics for voltage changes with fake wires or chargers.