I was trying to debug an ICE graph with Show Values, but the values would just not show. Eventually I turned to the docs, and I found my answer (in the Debugging ICE Trees > Displaying Port Values topic):
Per-component attributes cannot be displayed on polygon meshes that are subdivided using the geometry approximation method.
Sure enough, my mesh had a Geometry Approximation property on it.
Hat tip to the doc team for adding this info to the 7.5 docs (it wasn’t there in 7.0).