Job source repository not being polled for changes

I created a plan that has 3 jobs. Two of the jobs are configured to use a different source repository from the plan's source repository. I have the plan source repository set to trigger the build by polling the repository every 10 minutes. However, there is no such option for the job source repositories. Therefore, I would assume that job source repositories can trigger a build as set up in the plan's source repository configuration. However, this does not seem to be the case. Changes in the job source repositories never trigger builds. Furthermore, these changes never show up in the Code Changes section of the Build Result Summary.