Here are a few things to check/try:

(1) The most likely cause of such a problem is a conflict with a theme or plugin.
Try temporarily reverting to the default WordPress theme (Twenty Fourteen) to see if this helps.

(2) If not, then try temporarily deactivating all plugins other than WP-Juicebox and reactivate them one-by-one (checking WP-Juicebox's functionality after each one) to see if you can find the source of the conflict.

(3) Please try clearing your browser's cache and trying again.
This will ensure that your browser fetches and uses the most recent versions of the WP-Juicebox files from your web server.

(4) If you use the BulletProof Security plugin, then this may be the cause of your problem and you will need to add a skip/bypass rule to the .htaccess file in your root directory as follows:

# Juicebox skip/bypass rule
RewriteCond %{REQUEST_URI} ^/wp-content/plugins/wp-juicebox/ [NC]
RewriteRule . - [S=13]

(5) If you do not use the BulletProof Security plugin, then please check your .htaccess file for an entry such as:

<Files *.php>
    deny from all
</Files>

If you find such an entry, either comment it out or remove it.

(6) Also, try checking the permissions of the files and folders in the 'wp-content/plugins/wp-juicebox/' directory on your web server to make sure that they are not too restrictive. Default permissions of 755 for folders and 644 for files should be fine.

Hopefully one of these suggestions will help.

Ok so a quick test shows that the plugin works fine on a regular page using default template, but there is an issue with using the Page Composer template - which unfortunately I need to use to have my layouts the way I want.... any suggestions? Have you heard of this conflict before? thanks