Hello David, welcome to the Community!
When you make any changes to the Custom types and documents in your prismic repository, you need to then run
gatsby clean in your project to delete the cache and have the most up to date version of the schema.
Also, make sure you're using the correct browser tool to test your queries which is the built-in Gatsby Playground.
Prismic also has The GraphQL API Explorer. This is meant to test queries when working with GraphQL but not for a Gatsby website . The schema definition is different from the Gatsby schema and won't work to test your Gatsby queries.
It's important to mention that we've moved away from supporting/documenting this plugin. We have a dedicated article if you're interested in learning the details about this process: Gatsby-Prismic plugins: what’s going on?. We highly recommend that you migrate your project to
gatsby-source-prismic with the help of this guide: Migrate to the recommended plugin.