Why can I not change the color assignment on some models in GrabCad ?

Hi,

I have some trouble changing the material of some imported WRL files in GrabCad.
With some of my models I can, and with some others I cannot... But nothing in the not-working files looks different. The file seems to be OK, or at least GrabCad show no error and I find none in Rhino either.

What is the cause of this ? How can I avoid it and change every material of a WRL file in GrabCad ?

You will find one of the problematic file in attachment, along with a screenshot.