Have you taken the temp in your 'bator? Because temps are not always "true" just because the built-in thermometer says so. Even store-bought thermometers can be off. My point is, you could be running a bit cool and not know it unless you check with two or three different thermometers or at least a digital thermometer. Muscovy's take longer to hatch under ideal conditions. So just keep working on it. Meantime, there is a discussion of "cooling time" for incubating eggs on the Metzer website, pros and cons, I decided to do cooling phases and it lengthened my hatch time by at least 2 days. I also found out from my egg supplier that she incubates at 100 degrees F which is higher than the instructions on my 'bator and higher than what I found on websites. She has exactly 28 day hatches.
My nerves were completely frazzled by the time all the eggs finally hatched.