Why Does Search Volume Show Zero Sometimes?

We currently don't display search volume if you do not rank in the top 100 for those keywords.

Nozzle does not display search volume for keywords where the selected domain/brand doesn't rank in the top 100 for those terms or phrases.

We do collect all that information for those keywords, so it is still  possible for you to see the search volume.

Simply change the Rollup to Domain and select google.com (or some other Google tld).

Since Google has results on every SERP, this will display the search volume for (usually) all your keywords.