It's likely DNS. You need a forward A record and a PTR record before you deploy. I see lots of people missing the PTR. If you were missing a PTR record, and you simply add one, it won't magically start working, even if you restart vCenter.
However if you need to add your PTR record after-the-fact, you can simply Edit the network settings in vCenter Management (no need to make changes) and that will force the network settings to refresh, and then it will work.