DIAGNOSIS This is usually a battery communication or calibration issue, not a charging hardware problem. The robot thinks it's charging, but it's not actually accepting a full charge from the dock.
FIX First, let's try a hard reset to clear the communication glitch. 1) Pick the robot up off the dock. 2) Press and hold the power button on the side for about 15 seconds until you hear a shutdown sound and all lights go off. 3) Place it back on the dock, making sure the charging contacts on the robot and dock are clean and touching. Let it sit for a full, uninterrupted 4-6 hours. This often forces a proper calibration cycle.
IF THAT DIDN'T WORK The battery itself might be failing and not holding capacity. You can check this by manually starting a cleaning run after it says it's charged. If it dies almost immediately, it points to a bad battery that needs replacement.
HEADS UP Constantly running it from a low state of charge can permanently damage lithium batteries. If the hard reset doesn't fix it within a day, stop the cycle and look into a replacement battery to prevent further damage to the system.