I was having a problem with the "edit thumb" function of NextGEN and found many posts in these forums of people experiencing the same problem (the full size image on the left was not loading, and the cropping tool did not present itself). None of the potential solutions I saw in any of the threads had any positive effect on my situation.
In my case, after disabling all my other plugins, the problem was resolved, so I went back to the technique employed by that classic MacOS app 'Conflict Catcher' and systematically re-activated groups of plug ins until I narrowed it down to one culprit: WP Minify. I then tried adjusting the setting of that plugin to see what particular setting was causing the problem and it turns out that the mere presence of the plugin, even with all of its options disabled, was killing the edit thumb capability of NextGen.
I am going to try other plugins that provide the same the 'on-the-fly' compression of CSS as WP Minify to see if any will work alongside NextGEN. If I get any more information on this, I will follow up this post.