I found when I specified the geometry of the document, the changes package won't be displayed completely.
Here is my code:
\documentclass{book}
\usepackage{changes}
\usepackage[
a4paper,
top=3cm,
bottom=3cm,
left=3cm,
right=2.5cm,
]{geometry}
\begin{document}
text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text \comment{text text text text text text text text text text text text text text text text text text text text text text} text text text text text text text text text text text
\end{document}

