Is this a part that is only machined from one side? If it is I export my SolidWorks parts as DWGs or DXFs. I have imported IGES files into Mastercam many times and had issues with the geometry not being usable for toolpaths. I never have issues with DWG or DXF files.
Woops, did not see the date on the first post, may not be relevant anymore ...
Check to make sure your first radius is an arc. If it is a spline or ellipse, that could be the problem.
Are you 2D cutting, like waterjet or laser?