Your battery might be bad. Does the green light come on after about 10 seconds of being plugged in?
I had this same problem with my controller, it would only work plugged in even if the light was green, I would unplug it and it would work maybe 10 seconds. Also battery bar would never go over 3 bars even if the light was green.
Bought a new battery, charged fully before I used it and am now getting 20+ hours of play on it.
my problem is i can't TURN ON my xbox using the wireless pad with that button, BUT when i have it plugged in WIRED up, then i can use it...batteries r NEW so i know its not that issue, maybe its the cheap AFTERMARKET wired cable/hook up ?