I've read all sorts about this. Some say calcium deficiency and some say too much calcium is a problem and don't use oyster grit under any circumstances. We stopped using oyster grit 18 months ago, just ordinary grit used now. Made absolutely no difference to eggs. Most of our eggs are fantastic but odd one soft. I doubt that the level of calcium is the problem. Immunisation against ILT can cause some hens to regularly produce soft shell eggs -its a well documented side effect. Are your birds commercial hybrids? Some kind of trauma has the same effect on any of them. A fox around the coops at night and there will be several premature eggs with soft shells dropped from the perches. Bullying causes the same thing. We had a hen that was bullied and she laid soft eggs or none at all until the bully had a stroke and died. She is now the happiest , healthiest hen and lays fantastic eggs. Same with another who lost her feathers -they just fell out. Was nearly bald for 18 months then the bully died and she now has a full set of feathers. I'd check the pecking order as some of the lower order birds can be quite nasty to the lowest. Separation is the cure in that case.
