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.
Sean Cresswell Design Manager Streetscape Ltd Solid Edge ST10 [MP2] Classic [x2 seats] Windows 10 - Quadro P2000