Why wont my phone charge when on got power suply tap to change USB

It will only charge if it is turnd off

Welcome to Android Central! Which phone? When you plug it in, do you see a lightning bolt appear in the battery icon? If not, then have you already tried different cables and charging plugs?

Is this only happening when you're charging from a computer's USB port?

Remember, actively using your phone uses power, and if you're doing something that's processor intensive, then the rate of power usage might still exceed the rate of charging, especially if you're using a weak power source like a computer USB port.

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Which device ?
Make sure charger port is clean and cable fits snug,if it's a older device might have charger port issue and might need further diagnosis from repair center.