This usually indicates that there's a problem with the GitHub webhook, sometimes due to multiple being added to the project. The easiest way to get this working again is to have an admin of the Github Organization follow these steps in order:
1. Remove all CircleCI webhooks and services from your GitHub repo settings.
2. Tell the project to stop building on CircleCI through "Project settings".
3. Add the project back on CircleCI.
A full walkthrough for removing a project from CircleCI can be found here, including screenshots.