Plotting subfigures side-by-side?

Question

I am trying to plot two figures side by side and I simply can't find why my Output is stacking them on top of each other and not side by side. Any help would be greatly appreciated.

\documentclass[preprint,12pt]{elsarticle}
\usepackage{subcaption}
\usepackage{graphicx,caption}

\begin{figure}[t!]
\centering
\begin{subfigure}[b]{\linewidth}
    \centering
    \includegraphics[width=0.4\linewidth]{figures/coupling term graph}
    \captionsetup{width=0.4\linewidth}
    \caption{Plot of theoretical coupling term using the magnet and coil parameters given in table 1}
\end{subfigure}
\begin{subfigure}[b]{\linewidth}
    \centering
    \includegraphics[width=0.4\linewidth]{figures/coupling term superposition}
    \captionsetup{width=0.4\linewidth}
    \caption{Plot of theoretical coupling term using two identical magnets placed at each position for maximum flux for the coil parameters given in table 1}
 \end{subfigure}
\end{figure}

\end{document}

Answer 1

As per my comment. Note that [b] will align the bottoms of the captions, whereas [t] will align the bottoms (baselines) of the images.

\documentclass[preprint,12pt]{elsarticle}
\usepackage{subcaption}
\usepackage{graphicx}
\begin{document}

\begin{figure}[tp]% ! doesn't do what you think it does
\centering
\begin{subfigure}[t]{0.4\linewidth}
    \centering
    \includegraphics[width=\linewidth]{example-image-a}
    \caption{Plot of theoretical coupling term using the magnet and coil parameters given in table 1}
\end{subfigure}\hfil% equal to outside spacing
\begin{subfigure}[t]{0.4\linewidth}
    \centering
    \includegraphics[width=\linewidth]{example-image-b}
    \caption[b]{Plot of theoretical coupling term using two identical magnets placed at each position for maximum flux for the coil parameters given in table 1}
 \end{subfigure}
\end{figure}

\end{document}