When I replaced my stove hood the duct slipped down about 3/4 of an inch and would not go back up. A trip to the attic showed this was going to be a royal pain to fix without help. Put it off a couple of days and drank a few beers on it. I thought I would try something. I fastened in a piece of plywood the same size as the vent top and pulled it up with screws to apply pressure to the bottom of the duct. I tightened it down just short of causing the duct to buckle. Remove the paper from my vibrating sander and held the rubber pad of the sander so the side of the duct work. The vibration loosened it so it slid back in place.
Jimmie