The Button Bar icons are actually characters in a custom font so your problem is most likely a font issue.

However, if the problem is at server level (either incorrect or missing MIME type entries in your server settings or restrictive permissions for the font files in the 'jbcore/classic/fonts/' directory), then I would expect the problem to be seen across all platforms (not just on mobile devices).

If you are not already using the latest version of JuiceboxBuilder-Pro, then try upgrading your gallery to see if this helps.
Full instructions for downloading the latest version and upgrading existing galleries can be found here.

Even if you are already using the latest version of Juicebox-Pro, try re-uploading the gallery's 'jbcore' folder to make sure that all font files are present and correct on your server.

If you continue to experience difficulties, then please:
(1) Provide a link to your gallery so that I can try to see the problem for myself.
(2) Let me know what device and browser (including version numbers) you see the problem in.

Thank you.