I am also having the same issue but it happened when I updated the Next Gen Gallery from 1.9.3 to 2.1.23. I have since upgraded my WordPress to 4.4.1 but the slideshow broke with the plugin upgrade. Here is a page on my site that shows the thumbnails but when you click on one, it does not activate the slideshow. I have worked on the settings but that did not help. And now I am not sure the settings are correct anymore.
