Make sure the output current on the charger is at least around 1000 mA (1A). Older chargers that can't supply enough current will be supplying some of the current the phone is using, but not enough to supply current for the phone and to charge the battery too, so what you're doing is discharging the battery slower. If it's the stock charger, it could be a bad charger or a bad cable.)
Well, a good solution is getting a new charger, I can recomend a few ones, anker is a great one, I use it with my LG g flex 2 and nvidia shield, there are other options out there but that's my advice