I don't have a popup and never had experience with that model but with no other responses I might be better than nothing.
Is your crank turning (i.e. and not doing anything) or is the crank stuck?
I have two quick anecdotes from helping my grandpa with his popup.
First, there was the time his crank was stuck, I ended up crawling underneath and tapping/beating on the plate that moves along the long screw as he cranked. after the first couple inches it went down fine.
Second, probably because he never really fixed the first problem, his crank basically stripped. I ended up using a vice grip underneath to lower the top, 1/2 a turn at a time.