If CircleCI is not able to build when a branch is created or updates are pushed to a pull request, first of all, please check the GitHub WebHook deliveries as this might help in discovering why a push to your repo has 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 signing out of Github in your browser, signing out of CircleCI, then logging back into CircleCI via Github.
You can also follow the below steps to make CircleCI 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 feel free to contact CircleCI support.