Build times-out for a single user

We use CircleCI to build our upstream project and two user forks of that upstream project.

One of the user’s forks does not build. Their build times out at the end of the “test” phase.

All the sources are identical. The timeout occurs for builds after we’ve merged all pull requests and rebased all of the forks. We do use a custom “circle.yml”. They are all identical.

The circleci environments are identical, although I can’t be absolutely sure of this because key-values are obfuscated once set. I have tried resetting the environment for the affected user.

All of the repository sources are identical.

The build runs on the affected user’s local machine, and runs on my desktop when I check the affected user’s fork out.

What could be going wrong?