The power jack may need to be replaced, or the AC Adapter may need to be replaced. Use a multimeter to confirm that the power output from the AC Adapter matches what you see on the multimeter. If it matches, the by process of elimination, you can move on to the next possible cause.
After removing the motherboard, examine the power jack solder points, and the internal power jack for damage to the center pin. If you confirm the solder points are loose or broken; or the female receptacle area of the power jack is damaged, then replace the power jack.