is there any problem with the fbx automatic extraction of the .fbx models ?
i used that with lots of .x models and everything works good, when i changed to .fbx some models dont works correctly
The BEPUphysics demos all use .fbx files for models, so it shouldn't be a universal problem.
If you are deploying to a WP7 device, make sure you're using the latest BEPUphysics version compatible with the XNA 4.0 RTM.
Otherwise, the BEPUphysicsDrawer that comes in the BEPUphysicsDemos source has a triangle mesh drawer in it which is used to draw things like StaticTriangleGroups. You could use it to visualize the problem and see what it is doing.