Ensure that any 3D layers that you're exporting as U3D have only triangular object geometry. Additionally, while exporting 3D layers as U3D, keep the following limitations in mind:
- Higher-level primitives, such as NURBS, splines, and curves, are not supported.
- Texture mapping is limited to only one diffuse map per material. Ambient, specular, luminous, or opacity texture maps are not supported.
- Material animation is not supported.