Since this is not a temperature problem and you have had great hatches before hand, probably the reason many have died and are delayed is quality of the eggs. If they are more than 6 days old, it can be harder to grow, and also poor nutrition of the mother and father could be a result also. If the eggs were shipped to you, faulty transportation could also be a reason. There could be a number of reasons your eggs are not doing well, but whatever the reason, and this is totally your decision, an assisted hatch is needed to save the late, but still alive ducklings. They could give up at any time, and since they are so late and many have already died, now is the time to start.