Shadow generation has always been a little flaky, though it's gotten better since it was first introduced. Since you mention stairs, the problem may be with the shadow generation actually being turned off for complex elements (ie small polygons).
Do you have high accuracy turned on in the Photorendering Settings? I am not sure, but I suspect that this will also influence the vector shadows.
Shadow casting can also be turned off within the GDL script of library parts themselves. Check for a user parameter to adjust this. If shadows are turned off inside the script (without a user parameter) then the only fix is to edit the script.
