First of all, please double-check to see if your gallery is using Juicebox-Lite (the free version) or Juicebox-Pro.
Juicebox-Lite has an image limit of 50 images per gallery whereas Juicebox-Pro has no such limit.
If your gallery displays a Juicebox logo in the lower right corner, then it is a Juicebox-Lite gallery. (Juicebox-Pro galleries do not feature this branding.)
If your gallery is a Juicebox-Lite gallery, then instructions for upgrading existing galleries can be found on the Upgrading Juicebox support page.
Essentially, you can load and re-save your gallery in JuiceboxBuilder-Pro or replace your gallery's existing 'jbcore' folder with the 'jbcore' folder from the Juicebox-Pro v1.5.0 download zip package ('juicebox_pro_1.5.0/web/jbcore').
If your gallery is a Juicebox-Pro gallery and you use a Flickr account as an image source, you may need to increase the flickrImageCount from its default value of 50 to its maximum value of 500 on JuiceboxBuilder-Pro's 'Images' tab. (Incidentally, this maximum value is imposed by Flickr rather than Juicebox-Pro.)
Also, be sure to clear your browser's cache before reloading your gallery's page to ensure that your browser is using the latest versions of your gallery files from your web server (instead of perhaps using older cached ones).
I hope this helps.
However, if you continue to experience difficulties, please email me back with 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.