When create a new JuiceBox Gallery it doesn't appear in the grid

If you create a new Juicebox gallery page, it should appear on the gallery index page as soon as you drag and drop the Juicebox gallery page onto the gallery index page (on the 'Pages' tab) and click 'Save' and 'Publish', even if it is the only gallery page grouped under the gallery index page.
If you have not yet edited the Juicebox gallery page (for example to add images to the gallery), then the Juicebox gallery page will still be represented on the gallery index page but the gallery index page will just display a 'no images' icon (a white cross on a grey background) as a thumbnail image for the gallery.

If this does not seem to happen within your Showakse site, then maybe there's some kind of caching issue causing the problem (whether due to server-side caching on your web server or client-side caching in your web browser.)
Check with your web host to see if caching is used on your hosting account and try clearing your web browser's cache before previewing your gallery index page.

I can't be sure that this will solve your problem but I hope that it points you in the right direction.

Incidentally, I've tried unsuccessfully to replicate the problem you describe in a fresh Showakse v1.7.4 site.
A single Juicebox gallery page always appears on a gallery index page (whether I have uploaded images to the gallery or not) and without the need to add another gallery page to the gallery index page.