QUICK ANSWER
First test the dock from close range.
iRobot notes that a robot picked up and moved during a cleaning job can have difficulty finding home. A close-range docking test helps separate map/location problems from final alignment problems.

INTERACTIVE DIAGNOSIS
Where does docking fail?
STEP 1 OF 2
Was the robot or dock moved during or after the current map was created?
Manual relocation can make return-to-home behavior less reliable.
Avoid repeated map resets
Do not delete maps as the first fix. Confirm dock power, placement, clearance, camera area and contacts before rebuilding a working map.