first thing that come on my mind is credentials. Do you use the same credentials in VMAN as well as in the vCenter?
- I'd suggest that you login to vCenter MOB pages (https://vCenter_IP_addr/mob) with the VMAN credentials, go through the structure to focus on a cluster (with missing ESX hosts in VMAN) and check if all of the hosts are present
If it doesn't help then contact SolarWinds Support team. My guess is that the missing hosts have duplicated worldId, biosId or something like that.