Did you figure out this issue, because I am experiencing the same thing. Actually it is worse, because one of my VM Hosts reports hardware information and the rest do not. Working with Tech Support we did find a document on Vm Website that might help. i have not had time to read it, but maybe it will help.
Here is the URL:
HTTP://blogs.vmware.com/kb/2011/03/hardware-monitoring-in-esxi.html