I realize this is an old question and unsure if you ever found a solution.
Assuming we're talking about the fhe Pi-Hole DNS App (https://splunkbase.splunk.com/app/4506/) I encountered the same problem trying to install it today. Data is there, sourcetype set to pihole as outlined in the documentation.
What I found was my issue was that Acceleration for the DNS data model was not enabled. Going to Settings -> Data models -> Network Resolution (DNS) and enabling it for Acceleration solved my dashboards (enabling Acceleration may take some time to build depending on the amount of data).
Another option is to edit the dashboards and remove the "summariesonly=t" from the tstats queries. I assume all dashboards will run slower when summaries are not enabled - which was solved by the Acceleration for me.
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