What is the output in mA of the charger you are using? (It will be printed on the charger)?
It may be that the output is insufficient to run the phone as well as charge the battery effectively. At best it might just maintain the battery at its' current charge level or slowly increase the charge.
It might also depend on how much activity is going on in the phone. By this I mean what actions you are doing, (e.g. playing games, watching videos etc) and also what is happening 'behind the scenes', e.g are apps downloading in the background.
You could try turning off the Data Connection to see if this improves the charge rate with the phone still on. (don't know your phone but it is usually found in Home > Settings > Wireless and Networks >more >Mobile Networks > Data Connection. This setting disables the cellular data connection. Be aware that as well as disabling the cellular internet it also disables the phone's MMS feature (texts with videos and pictures) and also the LTE / 4G network phone connection. It does not affect SMS messages or 2G/3G phone calls.
If you need the data connection try disabling the apps in your phone ability to automatically update etc to limit unnecessary data transmission and therefore phone's CPU activity.
Also if you do not require them, turn off the GPS and WiFi / BT as these functions also use a lot of power.
Hopefully this is of some help.