I'm having the opposite problem to that article. Mine is constantly 'too light' because the unit/brush is constantly raised up, no matter what position the unit is in (upright, reclined, carpet, hard floor).
The servo motor is working because I took it apart and put it back in in the unit/brush down position (so heavy to move due to lots of engagement with the carpet), but after a short while it extended, raising the unit up, and it has stayed that way since. None of the lights come on (other than when it extended at the start).
In this fully raised position the unit is terrible at cleaning as it is too far from the floor.
Sounds like the sensor that detects that sets the level is malfunctioning perhaps?