take it off and put a spot of tipex on the spindle then switch it on and see the travel of the spindle with the mark on it - if it goes back and forth and stops in the same place re-fit the arm in the correct position and make sure the nut is tight
if it doesnt could be a faulty motor or a faulty earth
Erm I'll have to admit this one as "User error"; I could have sworn the arm rested towards the drivers side but I just found a picture of when I first bought it and it's resting towards the passengers side... as it always has...