It sounds like a wheelbox problem if the outer 2 wipers are working.
There are 2 nuts on the wiper wheelboxes backplate behind the dash that hold the wiper rack to the wheelbox drive wheel. You will have to remove these to look in side (and the nut on the outside of the car if you replace the wheel box).
Its likely that the wheel isnt connected to its shaft or alternatively the splined wiper fitting that the wiper attaches to has come away from the shaft. Car builder solutions or Melvyn Rutter will have the wheelboxes.
