I haven't seen this myself. Do you mean in the Materials room, you see only the names of the materials, but not the picture? Or, in the room itself you don't see the preview at the bottom of the main node? If the latter, check to make sure the eye in the upper right corner of the main node is showing it open. If the eye is closed, it turns off the preview. I had this happen to me after I accidently clicked on the eye, and it took me a little bit to realize what had happened.