Be sure to turn the input knob on the back of the keyboard, next to where the mic plugs in, clockwise to allow enough gain.
I assume you aren't using a phantom-powered mic. If so, you must turn on phantom power every time you power on.
Also:
Adjust the microphone volume by using the MIC VOLUME knob in the MIC/ VOICE section. You can see the volume of the Mic/In channel in the Main page of the Style Play or Song Play mode. Be sure it is not too low or muted.