Subfigure and figure exact place

Question

I have the following code. The issue is I got the one with subfigure above the figure one. What is the problem?

Wherein the T is temperature, $\varepsilon$ is the strain, w is an embedded FBG, and S2 is a number of sample. $w$ and $z$ indicates the FBG sensors in the middle (between the 3rd and the 4th layer) and on the surface of the sample respectively as represented in Fig. \ref{fig1}.
\begin{figure}[H]
    \centering
    \includegraphics[width=0.6\textwidth]{FBG_LOCATION.pdf}
    \caption{FBG sensor locations on the CFRP sample}
    \label{fig1}
\end{figure}
The thermal expansion coefficient is a first derivative of Eq. \ref{eq1} which forms a second degree polynomial. The strain vs temperature and Thermal expansion vs temperature is shown in Fig. \ref{fig2}(a) and \ref{fig2}(b) respectively.
\begin{figure}
    \begin{subfigure}{0.5\textwidth}
        \centering
        \includegraphics[width=\linewidth]{Strain-Temp.pdf}
        \label{fig2a}
    \end{subfigure}%
    \begin{subfigure}{.5\textwidth}
        \centering
        \includegraphics[width=\linewidth]{Thermal-temp.pdf}
        \label{fig2b}
    \end{subfigure}
    \caption{Strain vs temperature curve (a) Thermal expansion vs temperature curve (b)}
    \label{fig2}
\end{figure}