When the plugin fetches the applications in New Relic to fill the combo, it only brings the first 200 apps. As stated in New Relic documentation content returned by the NR API is paged. In the case of applications service, the page size is 200.  The plugin doesn't support paged content, it only retrieves the first page.

I made a PR with the fix, and I'm already using the fixed snapshot version in my jenkins instance:

https://github.com/jenkinsci/newrelic-deployment-notifier-plugin/pull/5