First of all if you added them by DNS name (not IP address) it may related to the DNS Service that are configured for the name resolution of your vCenter Server and your ESXi hosts. Check the related A records in the DNS records and also try to resolve the FQDN of each ESXi by the vCenter server (for example try to ping them in shell/SSH access) and also from the client computer too.
It can be an issue in the network connectivity between your vCenter and its related ESXi hosts.
Anyway you should check the following log files in the ESXi shell to gather proper information about your problem:
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