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 logout of GitHub and the login once again through CircleCI, i.e. sign out of Github in the browser, sign out of CircleCI, then log back into CircleCI via Github.
This should resolve your issue. If you are still facing the problem, please contact CircleCI support.