Why Does Search Volume Show Zero Sometimes?

We currently don't display search volume for keywords where the selected domain/brand doesn't rank in the top 100.

We currently don't display search volume for keywords where the selected domain/brand doesn't rank in the top 100. It's on the list for the dev team to fix.

In the meantime, you can see the search volume for all the keywords by changing the Rollup dropdown to Domain and selecting google.com (or other google tld). Since google has results on every SERP, you'll see the search volume for all keywords.