The 2 most likely causes are:

(1) A conflicting plugin.
(2) Custom CSS code on your web page (from either your theme or another plugin) which the gallery is inheriting and which is interfering with the gallery display.

First of all, try disabling all plugins (other than WP-Juicebox) and re-enable them one by one (checking your gallery's display after each one) to see if you can find a plugin which is causing your problem.

Also, try temporarily reverting to a default WordPress theme (such as Twenty Sixteen) to see if this solves the problem.

You could also use your browser's developer tools (usually accessed via the F12 key) to see if you can figure out why there is a black box where the gallery should be.

I hope these notes help.
However, if you continue to experience difficulties, then please post the URL to your gallery's web page so that I can see the problem for myself and hopefully help further. Thank you.

When using the Media Library as a source of images, WP-Juicebox displays images attached to the post containing the gallery.
The problem seems to be that your gallery is sourced by Media Library images but there are no images attached to the post containing the gallery.
Please double-check that there are images attached to your gallery's post.
Edit post -> Add Media -> Insert Media -> Media Library -> Uploaded to this post
If there are no images listed here, then there are no images attached to the post and you will need to upload some images for your gallery.

Please note that WordPress natively allows each Media Library image to be attached to only one post and it is not possible to attach an image to multiple posts.
You can use existing images from your Media Library but only if they are not already attached to another post. Otherwise, you will need to upload the images again for your gallery.