When a gallery is expanded on an iOS device (for example, from the Splash Page), the gallery is expanded on a new page instead of on top of the embedding page (please see the Expand Gallery Behavior support section) and Juicebox uses the 'full.html' file inside the gallery's 'jbcore' folder to display the gallery.

First of all, please check that the 'full.html' file is actually present inside your gallery's 'jbcore' folder on your web server.
If the file is there, then please check that its permissions are not too restrictive. Default file permissions of 644 should be fine. You should be able to check and change file permissions using a FTP program or via your web hosting account's online file manager.

Also, please ensure that your gallery uses the current version of Juicebox-Pro (v1.5.1) to be sure that any bugs from previous versions (but which have since been fixed) are not contributing to your problem.
If necessary, full instructions for downloading the latest version and upgrading existing galleries can be found on the Upgrading Juicebox support page.

Hopefully this will help.
However, if you continue to experience difficulties, please post the URL to your gallery's web page so that I can take a look at the problem for myself and hopefully help further.
Thank you.