I'm not an X-Plane user, but my best guess is the "missing" faces are flipped.
in AC3D, select your building and change the surfaces to 1-sided (the "1S" button in the lower left of the interface by "Surface type"). If it is a "flipped surfaces" problem, these problem surfaces should also "disappear" in AC3D (i.e., they'll only be visible from inside the building). If this happens, simply select the surfaces that are facing the wrong way and perform a Surface -> Flip Normal (Ctrl+Shift+N), and they should be fixed when you push the geometry out to X-Plane.
hope that helps,
Dennis
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