Soft shelled duck eggs

Does anyone have a solution to soft shelled duck egg please. I have at least 2 laying these regularly.
They are free ranging and have a pot of grit down which they don't touch

What kind of grit? I keep a feeder filled with oystershell and they fairly wolf it down. And you'd nearly want a hatchet to take the tops off the eggs!

It is oyster grit they have, maybe I will try moving it

Out of interest, Deryn, how d'you feed them? I keep two feeders filled with whole oats and another two with maize and one with oystershell and they help themselves whenever they feel like it.

It could be genetic. I have had this in the past when only feeding layers pellets. All the ducks were fine except one who laid soft shelled eggs. I guess that total free ranging might leave a deficit of calcium in their diet but as you offer Oyster shell supplement???

I have a feeder full of layer pellet all the time and the grit in a tray. Moved the grit today and noticed one of them tucking in. Hopefully the others will follow.