a) you are using the device while charging and draining more than it charges

b) You have a socket problem. With my charger, if I set it to charge and unplug the charger from the wall but not the jack from the phone, it never exits the charge mode because the residual charge in the charger and (maybe) because the voltage isn't sudden enough to trigger a removal - maybe it takes it as normal variation.

Try a better charger (different), the wall kind not the USB kind. If you have an European edition, use the adapter provided and charge from another charger.

Should the problem persist, there are some bugs to look at, followed by a reflash and finally by a trip to Nokia Care Center.

What is the software version?