Difficult to say what the actual issue is, without interogating the model data myself.....but I suspect there are one or more open regions that make this a surface model, rather than a solid. [a fully enclosed surface set]
Try running the "Show Non-Stitched Edges" from the "Modify Surfaces" group on the "Surfacing" tab.......this should indicate where the problem[s] is/are.
Something else to consider, is the import options may need to be adjusted to make it come in as a solid.....if it can actually be resolved by these settings.
If your still stuck, and if you allowed to, you may need to post the file here.