The recently added label whitelist/blacklist feature has an annoying bug: if you whitelist a label but forget to add that label when you first open the PR, adding the label later and commenting "test this please" still doesn't trigger the PR builder. The log contains entries like this

Mar 20, 2017 5:46:02 PM org.jenkinsci.plugins.ghprb.GhprbPullRequest checkWhiteListLabels
INFO: Can't find any of whitelist label.

even though the whitelist label is now set on the PR in question. The only fix appears to be to close and reopen the PR with the label in place to begin with.

This was also noted at #449 (comment)