As I’m often reminded while revising, when you’re persistently stuck, the problem isn’t usually in the scene you’re desperately trying to write, but two or three scenes back where you made a wrong turn or let a cat out of the bag that should’ve stayed there a little longer or a character did something totally implausible or out of character and you’re pretending it didn’t happen.  It’s like a leak in a sloped roof—the leak isn’t usually where the water’s coming into the house, but farther up the roof.  You keep bringing buckets to the problem when you need to get on the roof.
I suppose this is why I prefer to work recursively, looping back repeatedly, rather than a sustained march through a first draft followed by revision.