I don't know what is causing your problem but I do not think that it is a coincidence that the two images that do not display have the keyword "hidden" in their titles.
I have tried setting flickrShowTitle="FALSE" and flickrShowDescription="FALSE" to no avail.
I'd try actually changing the titles of these images to remove the word "hidden" (at least temporarily, for testing purposes) to see if this is, indeed, the cause of your problem.
If this does resolve your problem, then maybe you could use a term other than "hidden" (perhaps something like "secret" instead).
I'll try to figure out exactly what is causing this problem (whether it is a Flickr or Juicebox issue) to see if it can be resolved without changing your image titles.
I'll post back here once I've had a chance to investigate further.
Incidentally, I've just created a test Juicebox gallery using local images with the word "hidden" in their titles and captions and the gallery displays all images so, at the moment, it does not look like a Juicebox issue (at least not with local images).
I've also just added the word "hidden" to one of my own Flickr images (in the title and description) and a new Juicebox test gallery displays this image without any problem.
I'd certainly try removing all instances of "hidden" from your own images and, failing that, try reuploading them to Flick (as additional images and/or replacing the current images) to see if this helps.
I'll take another look at this later on when I have more time.
I hope you get on OK in the meantime. Please keep me posted with any updates on your troubleshooting. Thank you.