Jenkins github plugin isn't triggering a build when I push commits to master
<p>Also, I have a warning under the <code>Build when a change is pushed to GitHub</code> checkbox:</p>

<blockquote>
  <p>Hook for repo [redacted] on www.github.com failed to be registered or were removed. More info can be found on global manage page. This message will be dismissed if Jenkins receives a PING event from repo or repo will be ignored in global configuration.</p>
</blockquote>

<p>What are some possible causes?</p>