Parskip and Propositions

Question

I'm writing my thesis and I'd like the text to have some spacing between paragraphs. So I used

\usepackage{parskip}

The problem now is that, for some reason, it removes spacing in some parts of the text, where instead there should be some. For example, before a new Theorem or Proposition, as you can see in this example:

\documentclass[12pt, a4paper]{book}

\usepackage{parskip}    % paragraph spacing: without this command there is spacing between the paragraph and the proposition

\usepackage{amsthm}
\theoremstyle{plain}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{proposition}{Proposition}[section]

\begin{document}
\chapter{Introduction}
This is some text

This is other text in another paragraph

\begin{proposition}
This is the proposition
\end{proposition}

\end{document}

How can I prevent this?

EDIT: The parskip package also adds random space for no apparent reason in some parts of the text, see caption, even if the code does not imply those space to be there :

\section{Simulation and sensitivity analysis}
Some text some text Some text some text Some text some text Some text some 
text Some text some text
Some text some text Some text some text Some text some text Some text some 
text Some text some text
Some text some text Some text some text Some text some text Some text some 
text Some text some text

Some text some text Some text some text Some text some text Some text some 
text Some text some text
Some text some text Some text some text Some text some text Some text some 
text Some text some text
Some text some text Some text some text Some text some text Some text some 
text Some text some text

Answer 1

As this answer says: Using \begin{figure}[H], like any large box, is likely to produce bad page breaks with large amounts of white space. Try to avoid [H] and use [!htp] instead.

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The question was answered in the comments and, since no one wrote an answer, I made this answer a community wiki answer. This is to remove this question from the unanswered list. The original answer was by @egreg. The OP is encouraged to select this as the answer to remove the questions status as unanswered. If the person who answered in the comments decides to make an answer the OP can, and should, select that answer instead.