title: What can cause \ref{} of LaTex output a [??]
body: <p>I include a figure like this</p>
  
  <pre><code>\begin{figure*}
  \begin{center}
  \includegraphics[width=6.7in]{pic/recall_details.png}
  \caption{ Recalls of test query MB002-MB049}
  \label{ recall_details}
  \end{center}
  \end{figure*}
  </code></pre>
  
  <p>And then I refer it using <code>\ref{recall_details}</code>. Instead of getting numbers like 1, 2, I get ??. Literally I mean ??. What is wrong with my codes? How to make the references show correctly?</p>