Is the duckling thrusting outwards with its beak like it is trying to break the shell? Does it yawn or make nibbling motions? My chicken chicks do a lot of the eating/nibbling motion when they are still absorbing and not ready to come out yet, once they completely stop that and are frequently thrusting outwards with their beak I'll help them unless they manage to start zipping on their own. A lot of mine have started peeping more when they want out too.
Honestly I got all my best assisted hatching advice from Pyxis' article, paying attention to the nibbling/yawning has been a lifesaver for me!