Sometimes baby birds just do die when they are trying to hatch. Most of the time, there is not much you can do to prevent it.
You should make sure there is plenty of humdity in the incubator (it probably has a spot to put water: make sure that spot is full of water.)
Other than that, leaving them alone is usually the best thing to do.
Trying to help can actually kill the duckling, if you do it at the wrong time. They often make a hole in the egg when they still have a big chunk of egg yolk still attached to their belly. If you take them out of the shell then, they typically die. But if you leave them alone, they sit there slowly absorbing the rest of their yolk, and then they hatch after that (it may be 24 hours or more from the first hole to the actual hatching.)
https://www.backyardchickens.com/articles/guide-to-assisted-hatching-for-all-poultry.72886/
This article has lots of good advice, if you think you might need to help a duckling hatch. It talks about when to help and when to not help, and also tells how to help if that is needed.