When GitHub API answer from cache provider/github/watcher.go#L363

resp.Header.Get("X-From-Cache") == "1"

the project PRs are not listed for review because it is returned a NoErrNotModified error provider/github/watcher.go#L318

so no PR is handled provider/github/watcher.go#L198.

This is a problem if lookout didn't review that refs/pull/%d/head yet, but it is returned from cache by GitHub API. I think it can happen if the DB is dropped between reviews, or if the last review failed.