The usual reason for parametric analysis generating a flat line is that there is no clear data inheritance path from the building level (where the parametric changes are applied) down to the individual zones/surfaces.

In particular, when using Window to wall% as a variable, you should make sure that:

a) The Window to wall% value is not hard set at block, zone or surface level in positions in the building where you wish the parametric changes to be applied.

b) Likewise, you are not using custom openings, whose opening areas cannot be modified as part of the parametric variation.

c) You have one of the Facade types set that can accurately calculate window areas based on % glazing (Preferred height and Continuous horizontal) and that this data is inherited from building down to surface level.