I remember this happening to me. If I remember correctly I cured it by:-

• Exporting the prop.
• Delete the prop
• Re-import the prop.
• Click on another scene item
• Re-select the prop
• Parent the prop
• Save prop to library.

I forget the "why's?" if I ever found them out but it fixed it for me.

Also double check that you have an honest to goodness object not a carry over Rhino nurbs model.