HBO - Game of Thrones SPOILERS INSIDE. you have been warned

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[quote pid='203877' dateline='1556774140']
 Look how long the dead just stood there after they set the trenches on fire. That was prime time to dragon fire the hell out of them. They needed more of this. 

Dany was burning them the entire time they were stuck behind the trench and even beyond that up until the NK showed up.
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Not really, I just watched the scene and she attacked one time out away from the trenches after they lit them(granted, she may off camera been attacking out more). The dead just stood there until the NK told them to start making paths in the trenches.  They needed to light up the area around the trenches, keep that area saturated with fire at the choke points.