Virtually all of my 12 ducklings laid soft eggs first. I freaked out regularly for months. Even though they were all the same age, they were different breeds, They did not start laying at the same time. Not even close. It was two or so, months apart, Some of them laid soft eggs several times, a week apart, before they got regular. Some laid those quarter sized eggs that were perfectly hard, in-between soft eggs. I freaked out and did a LOT of research. (Needless to say, this was my first year with ducks)
My conclusion: Ducks are weird.
I knew they were getting ample vitamins & minerals & protein. But, because I was paranoid, I would sometimes mix some of the oyster shell, or egg shell right in with their food to make them intake more than they were eating. I don't think that was a great idea, but there ya go. I think they would have been fine without that addition, but it made me feel like I was doing what I could to get calcium into them. It probably does more damage than good.
Some of my research, at the time, indicated that ducks can have a genetic predisposition toward laying soft eggs and there is nothing you can do about that. Sorry I did not keep the references, but google knows, ask it to provide enlightenment if you feel that could be the problem.
Don't feel bad. Its is most likely not personal ;-) If you are feeding them a good quality feed and offering them calcium on the side, it is likely just an individual bird thing that will work itself out, or a genetic thing you can't do anything about anyway.
PS: for a great deal on oyster-shell calcium ask your feed store about the bulk bags. I think it's marketed for pidgins or something like that. I picked up a 50lb bag of premium shell, for under 20 bucks, where I was paying 12 bucks for one of those little bags by the fowl products on the shelf. It is not too bad to store it in a bucket along side their feed.