There are a few reasons why this might be happening.
Check out this thread, to see if it has any pointers to help you out. Re: VMWare Node Details Resources
If you do a MIB walk of your ESX server, you should see details of each VM on your server. We have a known issue where the vmConfigFile value is too long for our database. Increasing the length of your VMConfigFile column in the VirtualMachines table may fix this. This will be changed for SP1.
Otherwise, opening a trouble ticket might be the best way to go so that we can get diagnostics or have support walk through the steps of getting this fixed for you.