There are a few reasons why this is happening.
a) It could be your charger
b) It could be your cable
c) It could be an app or apps running in the back ground
Are you using the included charger and cable? After the phone has been on for a while, check to see which app or apps are using the most battery, and if they are running in the background.
If you are using a third party cable or charger, that could be the problem. Not all chargers and cables are of equal build quality.