The hexagon spinner you describe is displayed whilst an image is loading (and will continue to be displayed if an image cannot be loaded for whatever reason).
Please see this FAQ which might help:
My images show locally, but not when I upload them to my website. Why?
Try viewing the images you are having trouble with directly in your browser (paste the location of each image directly into your browser's address bar), bypassing Juicebox, to see if the images are displayed on their own.
If the images are not displayed, then, as well as the suggestions in the FAQ above, please check the following:
(1) Make sure that the images have actually been uploaded to your web server successfully and are in the correct location.
(2) Make sure that the images on your web server do not have permissions that are too restrictive and prevent them from being displayed. Default permissions of 644 should be fine. (You should be able to check and change permissions using your web hosting account's online file manager or a dedicated FTP program.)
I hope this helps.
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.
Once I am able to see your gallery live on your web server, I should hopefully be able to determine the exact cause of the problem and propose a solution.