My apologies for the late reply... I have been going through old unanswered posts to help other users.
This issue is quite common while importing from a different CAD software. It arises from the fact that some of the objects in the imported model are assigned the wrong type of element, namely FLOOR objects, when in fact they should be ROOF or CEILING elements...
The reason for this is that FLOOR objects do not usually cast shadows on the model grid since they are assumed to be elements already in contact with the ground. Importing from a 3DS file does not always convert the proper object assignment, and 3DS files import objects rather than layers (as in DXF) or materials (as in OBJ), so reassigning within the Import Window can be much too time consuming.
Before reconstructing the model in Ecotect, I would try to go to the Select menu > By Element Type > Floor, then assign the selected object to another ELEMENT such as WALL or PARTITION and see if that fixes the issue. You might then have to go back to actual objects that are supposed to be FLOOR and reassign them correctly.
Naturally, make sure your video card is up to date with OpenGl, and it might be a good thing to have Ecotect initialize OpenGl on startup, checking the option box under the User Control panel (Crtl+shift+P > Options).
I hope it helps.