I think that Alex's assessment is correct. Reduced scoring needs to be enabled both for the course and the set. Check both closely.
Here is a screenshot of the settings with the pertinent settings circled:
Here is a screenshot of what you should see if a reduced scoring set is not yet due:
and here is a screenshot of what you should see for a reduced scoring set that is in the reduced scoring period:
Those are both taken with the current version of webwork (2.20) on the problem sets page that students see.