AFAIK the random dungeon finder is designed to give you the same instance until either you complete the dungeon or the daily reset occurs. I imagine this is done to stop people gaming the system.
Alge
You have to drop group to get a new dungeon if you haven't cleared your current one. Make sure the eyeball that let's you port back to the instance (on your map) is gone. Although it is still not garaunteed to get something different.
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