Nextgen Gallery Images Not in Lightbox

I just started using the NextGEN Gallery plugin, and I’m having some problems.

One problem is that when I click on an image in a gallery that I’ve inserted into a post, it sometimes pops up in the lightbox style that it is supposed to. However, sometimes it doesn’t. It seems totally random. Sometimes it works as it is supposed to. Sometimes it doesn’t. I don’t see a pattern of any kind.

Any suggestions as to what is going on? Here is a post with a NextGEN Gallery gallery inside it:

Thanks.
Doug