Yes exactly landroverroy. I would feed more protein, and let them have more/longer free range time. A wild duck during its Moult needs about 1000 bugs/slugs a day to get enough protein for regrowth, so its not too different with domestic ducks. They need a lot of protein to actually have regrowth. Also during the regrowth period, try to avoid handling as much as possible. The new quills easily snap and bleed, and it just slows everything down
As well as mealworms and slugs, feed kale and parsley as well. they always seem to help a bit
Give them constant access to fresh clean water if possible, or at least twice a day, deep enough so they can wash off any feathers that dont preen out, and any in their noses...
Good luck