One possible problem is that you are using the wrong units. 3D models use units expressed in earth radii to express their size. So for example if you look at this example:
The scale value for the cube is 0.0002, which means 0.0002 earth radii.
If you want to convert earth radii to meters, you can use the Altus constant EARTH_RADIUS_METERS.
Another possibility is that you have made the model so large that your viewpoint is inside the model, and therefore you cannot "see" the model.
Another possibility is that you have located the model inside the earth (it will be invisible), or at such a great altitude that it is not visible.