The equation numbers do not appear in the PDF preview

In the PDF preview, the equation numbers do not appear.

When the following code is compiled, the PDF preview does not show (1) but (??). However, when the PDF is opened with Acrobat Reader, it shows (1) correctly.

\documentclass{article}
\begin{document}

\begin{eqnarray}
E = mc^2 \label{eq:emc2}
\end{eqnarray}

The equation (\ref{eq:emc2}) is already known.
\end{document}

[tavin]

Thanks @manmoi01 for the solution. For me c.LatexConfig.run_times = 2 was enough. This should really be mentioned in the README.

[manmoi01]

The solution to the problem is to add the line c.LatexConfig.run_times = 3 at the end of the file jupyter_notebook_config.py