Hi Cielia,
Shadow analysis is ArcScene is not very well supported.
You might to have a look at the Shadow Impact Analysis solution freely available for ArcGIS Pro.
http://solutions.arcgis.com/local-government/help/shadow-assessment/
Regards Gert
Gert van Maren
3D Solutions Engineer
C: +64 2040792466
gvanmaren@esri.com<mailto:gvanmaren@esri.com>
Solutions for 3D Basemap and Development Review<https://arcg.is/TfOOG>