I just tried exporting some very basic geometry as an IGS file (and also a STP file) into solidThinking Inspire and it failed to import. It was only one rectangle subtracted from another. So I am not sure what is going on there?
The exact same geometry works when exporting IGS/STP files using Rhino 5.
Hi Leslie - can you please post, or send me via private message,the Rhino file and results your getting from each, in STEP and IGES formats?
thanks,
-Pascal
No need!
I just installed the latest version of Inspire and it works! 
Not sure if it was that or because I had open polysurfaces.
Anyway, it works correctly now.
User error probably.