Since they are on chick crumbles, are you supplementing the food with niacin? If using chick crumb it needs to be supplemented 1 tbsp nutritious yeast to one cup of feed.
Pekins especially need a lot of niacin as they can end up with leg issues. If you haven't supplemented with niacin yet,
Are any of your ducklings pigeon toed or bow legged? If you arent sure, a picture of them would be helpful. Niacin deficiencies can cause decreased activity, they could stop eating, their legs become weak and grow pigeon toed.
Also do you give your ducklings treats? As too many treats can cause them to miss out on nutrients in their feed, and they should have chick grit available if they eat anything other than their feed.
I would get a thermometer asap so you can make sure they are the right temperature.
I'm really sorry to hear you lost one, sorry for your loss