After further investigation I have discovered this error is not dependent on the location or orientation of the hotspots but is the result of how the object is viewed in the 3D window. In this case, view the polygon from the top and all hotspots work fine. View it from a moderately tilted viewpoint and one or more hotspots may fail. View it from a very shallow angle and the hotspots will surely fail.
Geoff Briggs
DeForest Architects
Seattle, USA