It's covered by the picture form the next line as shown below:
I've tried to set subfigure with new width or new height, but the problem is still there.
The environment of my latex is a template on overleaf, I made a simple version and uploaded in Github here.
A simple version of the source code is as below:
\documentclass[type=master,fontset=fandol]{thuthesis}
\usepackage{thuthesis}
\graphicspath{{figures/}}
\begin{document}
\mainmatter
\chapter{test}
\begin{figure}[h]
\centering%
\begin{subfigure}{0.5\textwidth}
\centering
\includegraphics[width=\linewidth]{1.eps}
\caption{$v_l^* - l$}
\end{subfigure}%
% \hspace{4em}%
\begin{subfigure}{0.5\textwidth}
\centering
\includegraphics[width=\linewidth]{2.eps}
\caption{$\sigma_l^* - l$}
\end{subfigure}
\begin{subfigure}{0.5\textwidth}
\centering
\includegraphics[width=\linewidth]{3.eps}
\caption{$v_b - l$}
\end{subfigure}%
% \hspace{4em}%
\begin{subfigure}{0.5\textwidth}
\centering
\includegraphics[width=\linewidth]{4.eps}
\caption{$\sigma_b - l$}
\end{subfigure}
\caption{}
\label{fig:ov}
\end{figure}
\end{document}
\documentclass{...}
and ending with\end{document}
with your code fragment is not possible to reproduce your problem. – Zarko May 26 '19 at 9:21