Importing Errors - Some geometry disappeared!

When I have complicated models that I made with Cinema 4D R11.5, and I import them into my project, some of the polygons appear to be missing. I searched online for a solution, but no one seems to have the same thing happen to them. Please HELP!

Can you post screenshots? Do you get any import errors? Can you export manually from Cinema4D to FBX and import FBX back into Cinema4D without problems? Maybe it's just a problem with backface culling?