Hi. My hens are free range, access to grit, eat layers pellets and corn but a few weeks ago I still had a hen laying soft shelled eggs for over a week, sometimes no shell at all. I read ( perhaps elsewhere on here, but maybe not) that a shock can also cause this. This can be a snowfall, heavy traffic making a noise, anything that frightens her, even something you may not think of.
She's back to normal now thankfully
J xxxxx