After I initially setup Solarwinds, I noticed that one of our hosts wasn't listed in the overview section of the Virtualization manager. The only way I could get it back was to remove and readd the host. I came in today and found that only 3 of our 6 hosts are listed, the rest have disappeared. Why is it doing this?
Are you monitoring these hosts via vCenter? It's not recommended that you monitor hosts directly if they are managed by a vCenter. Also, are you using Virtualization Manager or the lightweight virt module inside of NPM/SAM?