DIAGNOSIS Dirty or corroded metal contacts on either the dock or the robot are breaking the electrical connection. Dust and fine metal shavings from the brushes are common culprits.
FIX Unplug the dock. Use a dry microfiber cloth or a pencil eraser to vigorously clean the four shiny metal pins on the dock's ramp. Then, clean the two matching metal contacts on the underside of the robot. Make sure there's no black film or grime left. Plug the dock back in and manually place the robot on it.
IF THAT DIDN'T WORK The dock's power supply might be failing. Check the white LED light on the dock is solid when plugged in (not blinking). If it's blinking or off, try a different outlet and check the power cable connections. A faulty power brick is a known point of failure.
HEADS UP Keep the dock in a relatively clean, dry area. Vacuuming around it regularly can prevent dust bunnies from coating the contacts again.