That's odd... I've just uploaded some images to a completely empty Flickr account, created an album/set (using just a couple of the uploaded images) and used the album/set id (grabbed from the URL) as the Flickr Set Id in JuiceboxBuilder-Pro. The album/set displayed as expected.

The only thing to note is that it took a good 5 or 6 minutes after uploading the images before they actually appeared in my Photostream but everything worked fine without a hitch.

The functionality certainly seems to be working OK (art least for me) but there are a couple of things you can do to try to troubleshoot your problem.

The first thing to do is to make sure that your images are public (and not private).

The next thing to do is to head on over to Flickr's own API Explorer (link below), enter your album/set id into the 'photoset_id' box (the second one down) and click the 'Call Method...' button.
There should be one <photo> tag for each and every image in your album/set shown in the output box.
If there are no images found, then there's definitely something wrong with your album/set at Flickr's end (if Flickr cannot find your images using the album/set id, then JuiceboxBuilder-Pro will not be able to do so either) and you might need to contact Flickr's own support team to figure out what's wrong.

Flickr API Explorer - flickr.photosets.getPhotos: https://www.flickr.com/services/api/exp … .getPhotos

I hope my notes above at least reassure you that the functionality should still be working OK (as it does for myself) and point you in the right direction.
If you continue to experience difficulties, then please let me know and I'll try to help further. (Are your images super-small or super-large? Maybe their size (in both filesize and/or resolution) has something to do with this? Perhaps you could try again with different images to see if this makes a difference.)