Using version 1.5.2 of Responsive Lightbox, and in fact the last couple of versions (I’ve been waiting to see if it would get fixed), the captions visible in the lightbox only show when viewing the gallery generated lightbox view. If you have a few posts in content, and they are viewed in the lightbox, then the captions do not show. See here for an example:

http://wshg.net/for-the-garden/2015-06-08/repurposing-a-recipe-for-charming-containers-full-of-plants/

Also, when long captions are shown they are truncated, when they should wrap if possible.

https://wordpress.org/plugins/responsive-lightbox/