You may also want to provide some more details, like for instance which version you are running of vCenter and ESXi. This could be various different things, but it seems like your management network interface on some of the hosts are not reachable. So I would start with going to the command line and try to ping each management interface from each host. So you do:
go to host 1, ping management ip of the rest of the cluster
go to host 2, ping management ip of the rest of the cluster
go to host 3 etc