The Chain Gun And You
 

Just did the lvl73, Dragonblight, "The Chain Gun And You" quest. Needed to turn off all things nUI in order to get the two necessary buttons.

I did that quest with nUI, no problem. Are you sure you have your main action bar on page 1?

Absolutely.

Hmmm... okay... I have a toon in that zone now... I'll see if I can't get him to that quest this evening and find out what's going on.

Similar issue
 

With the quest in Icecrown where you shoot the Proto-Drakes down for Ebon Knights rep. Couldn't get the shooting button or the energy restore button to work. Buttons showed up, but no firing.

Same sort of issue with the Blight Crystals quest in Zul Drak - no pet bar on my paladin (is supposed to show up where pally auras are). Later doing quests in that same line I did have pet bars kinda smooshed in the area where the auras are but they were sporadic in their working. I know the Crystals quest is buggy with pet loving toons like hunters, you have to dismiss your own pet first to get the pet bar to show, perhaps this is a blizz problem, not sure.

Also, the quest Aces High - combo points do not show on the target and the exit the vehicle button does not appear in the little boxes - you can sEE the box where it goes, but not use the button.