Take a look over at https://answers.splunk.com/answers/356938/sitename-key-causing-failed-dashboard-searches.html?childT.... If your lookup_host_site.csv has values, and SiteName is what's causing PerfMon data not to show, it could be case-sensitivity.
In the dashboard source query:
Change lookup_sites.csv to lookup_host_site.csv.
There is no lookup_sites.csv in the app.
Also, by default the lookup file won't have much data. You have to add to it as per your environment.