Branches not building

I’ve just set up CircleCI, and I’m only getting builds when I merge to master, not when I push to other branches.

I don’t have a circle.yaml config file, so I expect the default behaviour to be as described in the docs.

I’m not seeing the webhook in$ORG/$REPO/settings/hooks as suggested by this thread. Unfollowing/refollowing the repo doens’t populate the webhook. Adding the CircleCI webhook manually from Github doesn’t work either.

Of note is that this repo is a fork of another private repo, from an org that I control. That repo is building correctly on branches, but I want to be able to test my personal branches before opening PRs to the main project.

Am I missing something obvious, or is this behaviour bugged?

After adding a circle.yml file to set the python version to 3, the fork has started building again.