How long that vROps is running? Take in mind that it take some time (up to 24 hours) to shows up.
I would also check the time settings in the policy. Just ensure that it's either 24x7 or if you use specific dates that there is at least one day checked.
Was this ever collecting capacity metrics? If so, what happened on the day it stopped? It's never a bad idea to take the cluster offline via the AdminUI and bring it back online. That might kick vROps into gear.
I discovered that my custom settings for Capacity calculation suddenly had disappeared.
I originally selected Monday to Friday in the Time Range.
After some time all of my clusters had question marks.
There is probably a bug that whipes the custom Time Range when something else has been changed and saved in the policy.
I have an open PR on this issue.