Unfortunately, your problem may be caused by a known issue whereby Flickr descriptions are not always displayed when certain configuration options are set to specific values.
The problem may occur if you set showThumbsOnLoad="FALSE" or maxThumbRows="0" or maxThumbColumns="0".
This issue has already been logged as a bug and should hopefully be fixed in the next version of Juicebox-Pro.
In the meantime, possible workarounds would be to either:
(1) Do not use the configuration options values noted above.
... or:
(2) If your gallery is not too large, try setting imagePreloading="ALL".

If none of these suggestions work for you, then please post back with the URL to your gallery's web page so that I can investigate further. Thank you.

Thank you for providing the URL to your gallery's web page.

It looks like the problem is that there is not enough room in your gallery's caption area to display the Flickr descriptions along with the Flickr titles.

When setting captionPosition="BELOW_IMAGE" (as your gallery does), Juicebox uses the maxCaptionHeight value as the actual height of the caption area (rather than a maximum value). (This is noted in the short description for maxCaptionHeight in the Caption Options section of the Config Options page.)

Your gallery currently sets maxCaptionHeight to "50px" which is too short to accommodate both the Flickr titles and descriptions.
Increase maxCaptionHeight (perhaps to its default value of 120) and you should see the Flickr descriptions below the Flickr titles.
(Incidentally, the value for maxCaptionHeight should be just a numeric value, without the 'px' suffix.)

I hope this resolves your problem.