I have seen this issue and cannot remember the solution. In fact, I tried to repeat it with a draft dim. I thought that was the issue. Have you tried to load a new setup.dtl?
Is it a part dimension or a dimension created in the drawing? You can only change the nominal if it is a dimension driving the geometry, not a dimension created in the drawing.
Somehow my mind was stuck on changing the tolerance. If you right click a dimension and do not see "Nom Value" appear, it is a driven dim and it's nominal value can only be modified by changing the model. Dr. is correct again