I could have written this thread myself! My Pekin Duck has also been laying soft shelled (or actually shell less) eggs for too long. I tried giving Calcium Citrate + Vitamin D3. Then I increased it to two tablets a day. I'm still having no luck. She doesn't lay much anymore and that's really worrisome. Like OP mentioned, Honey Boo Boo has always gobbled up the oyster shell like it's candy. Since I have 2 drakes in with the ducks, I worry about feeding layer feed. I tried giving her a special treat of layer feed in multiple ways and she wants nothing to do with it. Unfortunately now I'm unable to get their normal feed since the feed mill burned down, so my big ducks aren't too happy. I'm following along with this. I think my next step is getting some calcium gluconate and doing half Calcium citrate + D3 and half a dose of calcium gluconate. I worry that she'll end up with too much Calcium since I don't have a vet that will even consider seeing poultry. I've thought about an antibiotic so I'm curious about how this goes with your duck @Mollieb713 In the back of my mind, I'm worried that her shell gland is damaged, maybe due to having so many antibiotics in 2020. That's probably not possible for the two to impact one another but I think we all think about what we've done in times like these.
Edited to add: It's so crazy how similar this is. I've also been giving dried black soldier fly larvae! They love it, but it's done nothing that I can see for Honey Boo Boo shell issue. I've given my duck Baytril on multiple occasions. It's not advised in poultry, so I should put that out there as a side note because I usually end up getting a reply about that.
Edited to add: It's so crazy how similar this is. I've also been giving dried black soldier fly larvae! They love it, but it's done nothing that I can see for Honey Boo Boo shell issue. I've given my duck Baytril on multiple occasions. It's not advised in poultry, so I should put that out there as a side note because I usually end up getting a reply about that.