There is a proximity trigger for when you need to speak with Zima while she is inside the cage.
If you finish killing the nearby mobs before going to the cage you will not be able to speak with her or continue the story.
(This will cause a lot of confusion and problems going forward if this isnt addressed so that Zima will speak to you even if you dont go to the cage directly.)

Once that trigger goes off and Zima begins to speak you must go immediately to the cage or else the option to speak with Zima to continue the quest will not be available.

Took me three tries resetting the zone to figure it out.
(EDIT: Suggest that perhaps the area of the proximity trigger is too large, Shrinking it to just around the cage may be an easy fix. Currently it can be triggered without the cage even being within site of your character.)