Your everyday USB 2.0 port is all of 0.5 Amperes. And if your phone is on, it charges slowly to maybe not at all. This effect has thousands of prior discussions so I won't duplicate why here.

http://www.cnet.com/how-to/use-ampere-to-ensure-your-android-device-is-charging-properly/ notes a nice app to see more detail and a link to how to speed up charging. For me that's a simple "put the phone into airplane mode." There are other tricks and tips out there by the web load.
Bob