First of all, please check to see if your gallery is using Juicebox-Lite or Juicebox-Pro. (Juicebox-Lite galleries are limited to 50 images per gallery but Juicebox-Pro galleries are not.)
A Juicebox-Lite gallery will have the Juicebox logo in the lower-right corner. There is no such branding in Juicebox-Pro galleries.
If your gallery is still Juicebox-Lite, then you can upgrade it to Juicebox-Pro by either:
(1) Opening the gallery in JuiceboxBuilder-Pro and re-saving it on the 'Publish' tab.
... or:
(2) Manually replacing the gallery's existing 'jbcore' folder for the version from the Juicebox-Pro v1.5.1 download zip package ('juicebox_pro_1.5.1/web/jbcore').
After upgrading a gallery, be sure to clear your browser's cache before reloading your gallery's web page to ensure that your browser is not using older cached files.

If your gallery is sourced by a Flickr account, then you may need to increase the value of the flickrImageCount configuration option (on JuiceboxBuilder-Pro's 'Images' tab) from its default value of 50 to something higher.
Please note that when using Flickr as a source of images, the maximum number of images which can be displayed in a Juicebox-Pro gallery is 500. This limit is imposed by Flickr rather than Juicebox-Pro itself.

I hope this helps.
If you continue to experience difficulties, then 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.