Your gallery's height is currently expressed as a percentage.
This will be a percentage of the gallery's parent container but, when the page is first loaded, the gallery's parent container is not visible and Juicebox may see its height as being 0px. 60% x 0px = 0px which may explain why the gallery is not displayed.
Instead, try expressing your gallery's height as a fixed pixel value (e.g. 600px) to see if this makes a difference.
If this does not help, then try also adding the gallery's height as inline CSS to the gallery container div, ie.:
<div id="juicebox-container" style="height: 600px;"></div>
... similar to the 'Using Percentage Heights' suggestion here.
Also, as you have multiple galleries which may be displayed on the same page, make sure you embed each gallery into a container with a unique id.
For example:
<script type="text/javascript" src="http://www.monpetitblog.net/galerie/2013_11_juice/jbcore/juicebox.js">
<script type="text/javascript">
new juicebox({
baseUrl : 'http://www.monpetitblog.net/galerie/path_to_gallery/',
containerId : "juicebox-container1",
galleryTitle: "Sept.'13",
galleryWidth: "100%",
galleryHeight: "600",
});
</script>
<div id="juicebox-container1" style="height: 600px;"></div>
<script type="text/javascript">
new juicebox({
baseUrl : 'http://www.monpetitblog.net/galerie/path_to_another_gallery/',
containerId : "juicebox-container2",
galleryTitle: "Sept.'13",
galleryWidth: "100%",
galleryHeight: "600",
});
</script>
<div id="juicebox-container2" style="height: 600px;"></div>
Incidentally, as you have WP-Juicebox activated, the 'juicebox.js' JavaScript file will be automatically loaded on your pages so there is no need to include the file in each and every gallery you manually embed.
(If you were to deactivate WP-Juicebox, you would need to include the file in each gallery's embedding code.)