Losing thin polygons in models

[Alverndbl]

Are you sure the polygons that you say they are missing don't have the normals reversed in Blender? If they appear in other software probably that's the problem. Try to select the "missing" part or press A in Blender edit mode and see if the missing part is highlighted. If that's the problem go to the Mesh menu in the 3d view tab then Normals and play with that. (If you selected all probably Recalculate outside will work, if only the specific poly the Flip)