[users at bb.net] How to debug why the buildmaster won't start a build?

[alan somers]

I have a buildmaster with one DockerLatentWorker and two builders.
Each builder has two change sources: a GitPoller and a
GitHubPullrequestPoller.  Each builder also has two schedulers: an
AnyBranchScheduler for a few select branches and an AnyBranchScheduler
for pull requests.  I also have Github hooks setup to push commit
notifications into the buildmaster.  There is a WorkerLock with a
maxCount of 1 that is shared by one step of both builders'
BuilderConfigs.

Everything was working fine until today, but now the buildmaster never
attempts to kick off a build.  I can see from twistd.log that the
GitHub hook is still working, and I see log entries like "added
buildset 183 to database".  However, whereas normally that entry would
be followed up by a "starting build" entry, now there is nothing.
There aren't any errors; there's just nothing at all.

How can I debug this problem?  I have Manhole working, but I don't
know what to look for.  Any advice would be appreciated.

Buildless in Boulder,
-Alan