I use the github branch source plugin with a github.com hosted organization

The organization setup is:
Discover branches: "Exclude branches that are also files as PR"
Discover pull requests from origin: "Both the current pull request revision and the pull request merged with the current target branch revision"

I recursively updates sub modules and checkout over SSH

When a PR is created, the two pull request jenkins jobs "PR-head" and "PR-merge" are created, but Jenkins does not start them

The "Organization event" log shows messages:
> No automatic builds for PR-XX-head
> No automatic builds for PR-XX-merge

Why are the PRs jobs not built ?

Thank you very much