Try do this:
In Lightbox Plus settings Advanced tab, enable "Use per page/post options:" Then you tick all 3 boxes, well the other 2 besides the one you have done already. I think this is funky part of Lightbox Plus right now :)
And now it works? I need to do this anyway. Your code is correct, images pop up as they should after this ticking exercise.
I use beta 2.5.6 http://wordpress.org/extend/plugins/lightbox-plus/developers/ so that might have something to do with it.
I had a bad feeling I had forgotten something. And now it arrived to part of brain that is awake. What I think I meant by mentioning those boxes is difference in behavior on single post vs. a loop/archive. Like in a category. Works in single, not so much else where. Full content or read more/excerpt make no difference. And this is actually what is no longer an issue with beta version but is with 2.5.5