Build does not kick off when GitHub PR opened

I work on a few private GH repos that have a CircleCI webhook that triggers builds only on the “Pull Request” event (which includes the PR being opened). However, a build is never kicked until a post-PR commit is made to the branch. Until that happens, all I see on GitHub is ci/circleci — Waiting for status to be reported, and I don’t see any activity on the CircleCI UI. Once I make another commit, there will be a build with each new commit as expected.

Is there something I can do to fix this? I’d like to avoid needing a “bump commit” with each PR.

For others, I was able to fix this problem by checking the “Only Build Pull Requests” box in Builds > Advanced Settings, which is probably a unique issue given the GH-side settings.