Cannot get my figure placed at the right spot

Question

Although I use the option [H] or small case analogue of it [h], my figure is placed separately from the text how can I solve this issue? I tried to provide a minimal working example but this may not illustrate my problem since the document is much more complex than this cut-down version. The minimal working example is:

\documentclass[11pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[a4paper]{geometry}
\usepackage{graphicx}
\begin{document}
\subsection{Soliton Evolution}
We begin our analysis by simulating the evolution of a third order soliton in the PCF Thorlabs NL-PM 750. This procedure of investigating soliton evolution is usually employed to test the accuracy of the code since solitons are analytical solutions of the NLSE, that is Eq.(\ref{eq:NLSE}).\cite{3} In order to conform with the analysis in the thesis and the article by the Dudley group we select the parameters corresponding to a third order soliton in our fiber\cite{thesis, 6}:
\begin{equation}
\begin{split}
L_D &= \frac{T_{0}^{2}}{|\beta_2|}= 0.158242\ \si{\meter}\\
L_{NL}&=(\gamma P_0)^{-1}= 0.017602\ \si{\meter} \\
P_0& = 598\ \si{\watt} \\
N &= [L_D / L_{NL}]^{1/2} \cong 3 \\
Z_{soliton}&=(\frac{\pi}{2})L_D= 0.248566 \ \si{\meter}
\end{split}
\end{equation}
\begin{figure}[H]\capstart
    \centering
    \includegraphics[scale=0.5]{SolitonEvolution(3rdOrder).jpg}
    \caption{The evolution of a third order soliton with the central wavelength of 790 \si{\nano\meter}.}
    \includegraphics[scale=0.5]{SolitonEvolution(3rdOrder)Solperiod.jpg}
    \caption{The same soliton with the length scale expressed in terms of the soliton period.}
\end{figure}
\subsection{Soliton Fission}
When we introduce the Raman scattering into the NLSE by modifying it as follows:
The parameters selected correspond to a third order soliton and is the same as before.
\end{document}