I had this issue once - there is an app on the Play store called "Galaxy Charging" that will tell you the amperage of the charging. If you're using the supplied Samsung charger (the white one), you should be getting 1800mA. If you're not, there's something wrong. I hadn't rebooted my phone in a very long time when this happened, so the first thing I did was reboot the phone.
Went from 450mA to 1800mA as soon as it came back online.
OH - and one of my other issues was that the USB blade wasn't completely down inside the wall plug.