I'm sorry to read about your precious ducks laying troubles. Like many others, I've been in your shoes with my Pekin over the past year and a half to two years. I've almost decided that my ducks shell gland just isn't functioning anymore. I continue to feed a layer feed because my Buff duck is laying and I'm hoping this will help my Pekin to lay her membrane eggs. I've tried calcium gluconate and calcium citrate(+D3) but find the citrate is much easier to give, in my personal opinion, with this particular duck. I was really hoping she'd start to lay nicely shelled eggs again after her winter break, like many people have noticed with their ducks, but this hasn't been our luck. I don't think this is a normal outcome with these ducks, but it'd be a good idea to stock up on a few broad spectrum antibiotics to have on hand, just in case.
Of course, I hope and pray Sprightly begins to lay those nice perfectly shelled eggs again very soon and for the rest of her laying days