How can I troubleshoot a tin that won't import into a terrain model?

Using Power GEOPAK SS4 08.11.09.872 on Windows 10.

When using the terrain model create from file command, my tin won't import - starts "processing" and never finishes. A different (and larger) tin imports in seconds. I can use the old dtm tools to visualize the tin, and my workaround is to load the tin triangles and tin hull and create a terrain model from those elements. I can use the terrain model properties to control how the terrain model is visualized, though I can't assign a feature definition to the terrain model created in this way.

I presume there is something wrong with the tin file but am not sure what to look for.