Your gallery is sourced with images from the Media Library and WP-Juicebox will display all images attached to the post containing the gallery. It looks like there are only three images attached to the post containing your gallery.

Try the following to check how many images are listed as being attached to the post containing the gallery.

• Edit the post containing the gallery.

• Click the 'Add Media' button.

• Select 'Insert Media' from the menu on the left (selected by default).

• Select the 'Media Library' tab (also selected by default).

• Choose 'Uploaded to this post' from the drop down menu.

• See how many images are listed.

You could also go directly to your Media Library and check which images are listed as being attached to the post containing the gallery (in the 'Uploaded to' column).

You should be able to add images to an existing gallery by following the Editing a Gallery instructions on the WP-Juicebox support page.

(1) Are 3 or 10 images displayed in the 'Uploaded to this post' section when you edit the post containing the gallery and click on the 'Add Media' button?
(2) Are 3 or 10 images listed as being attached to the post containing the gallery in the Media Library itself?

If you are unable to attach more that 3 images to your post, then this is a core WordPress issue (and not directly related to Juicebox).
The process of uploading images and attaching them to posts is done purely by WordPress itself. Juicebox simply displays whatever images are attached to the post.

You should be able to attach an image to a post with the following process.

• Edit the post containing the gallery.

• Click the 'Add Media' button.

• Select 'Insert Media' from the menu on the left (selected by default).

• Select the 'Media Library' tab (also selected by default).

• Drag and drop an image (or images) into the media window.

• Close the media window.

If this does not work, then you should be able to go directly to the Media Library, upload an image (using the 'Add New' button) and then attach it to the post containing your gallery (via the 'Attach' link in the 'Uploaded to' column).

If neither of these processes work, then you might need to search through (or post in) the WordPress support forum for the problem "unable to attach images to post".

There is a distinction between inserting images within a post and attaching them to a post.
Images need to be included in the Media Library and attached to a post to be displayed in your Juicebox gallery.

I have been unable to replicate the problem you are experiencing but if you allow me access to your WordPress installation, then I would be happy to take a look (and create a test gallery) to see if I can determine the cause of your problem.
If you are agreeable to this, please post back to let me know and I will send you a message with an email address which you can use to send me your WordPress installation login details.