If CircleCI is not able to build when a branch is created or updates are pushed to a pull request, first of all, check the GitHub WebHook deliveries as it will help in discovering why perhaps a push to your repo has potentially not resulted in a job on CircleCI.
If you are seeing the response 400 in spite of having the read/write/admin access to the repository, please try to completely re-authenticating, i.e. sign out of Github in the browser, sign out of CircleCI, then log back into CircleCI via Github.
You can also follow the below steps to cause CircleCI to generate a new webhook:
- Unfollow the project from the CircleCI ‘Project settings’ page - click the ‘Unfollow’ button
- Remove the webhook for the affected repo on GitHub (Settings > Webhooks and services)
- Re-add the project by following the project again on the CircleCI ‘Project settings’ page
This should resolve your issue. If you are still facing the problem, please contact CircleCI support.