Objects that are used in the Geometry or Fire Geometry LOD must be closed and convex in order to work.

To test for these qualities you can use the following methods:

Open Objects

Either select the objects manually or find them via
Structure → Topology → Find Non-Closed

Then close them via
Structure → Topology → Close

Non-Convex Objects

Either select the objects manually or find them via
Structure → Convexities → Find Non-Convexities

and then close them via
Structure → Convexities → Convex Hull

Or, alternatively, apply a
Structure → Convexities → Component Convex Hull

on all objects in the current LOD

Faces/Textures

In general it is also a good idea to test for any illegal/dead textures via
Structure → Check Faces