I would candle them to see if they have internally pipped yet. If they have and you can see their little beaks in the air sack, wiggling and moving around, give them time...personally I've never helped one unless it has externally pipped and is struggling really badly. What kind of duck's are they? Muscovy's don't start hatching till day thirty. You also might not have kept the incubator warm enough and they are just developing slowly. Give them time.
Also, if you do break down and decide to help them, DO IT SLOWLY, let them do most of the work and all of the pushing, DO NOT PULL THE DUCKLING OUT!!! Break along the edges and leave the top intact, LET IT KICK AND PUSH ITSELF OUT... I DO NOT SUGGEST HELPING but if you must, take your time and do it right, hatching is a process that the ducklings need to go through in order to be strong enough to live. Making one tiny hole in the shell for them to breath, only if they have internally pipped and there is no chance of messing with the humidity, should be the most you have to do. I would do one egg as a test run, if its worth the risk to you...