Diagnosis
The metal contacts can look clean but have an invisible film that prevents a good electrical connection. This is super common in homes with hard water or where floor cleaners are used frequently.
How to Fix It
Power off the robot. Get a pencil eraser—the classic pink kind works best. Gently but firmly rub the four metal charging contacts on the underside rear of the robot. Do the same for the three corresponding contacts on the front of the Clean Base docking ramp. Finish by wiping all contacts with a dry cloth to remove any eraser debris.
If That Didn't Work
Check the physical alignment. The robot might not be driving all the way up the ramp. Ensure the ramp is perfectly flat on the floor (no rug lip pushing it up) and that there's no debris on the ramp itself. Also, check the robot's front wheel for hair wrap, which can affect its driving height.
Heads Up
Never use liquid cleaners or metal polish on the contacts. You'll accelerate corrosion. The pencil eraser trick is the official, low-tech fix for a reason—it works without damaging the plating.