Does this go away itself after a few minutes (or on refresh); or does it seem to stick around for a while.
I ask as I have seen two things that causes this for us (I am sure more exist).
1. Refresh solved; if a thumbnail is not in cache VCD does not go get the tumbnail then reutrn. It returns; then goes an gets thumbnail for next request.
2. ESXi host that the VMs are on might need to be unprepared and prepared again. (If this is the issue I would expect that some hosts show thumbnails and others do not)
I am sure this is far from a complete list, but at least a couple things to check to start.