I am also getting this problem this week. Yesterday there were 2 eggs without shells in the coop when I let the girls out. Later there was another one in the nestbox along with 4 good eggs. This made it seven eggs and I only have 6 chooks. This morning there is another shell-less egg. Don't know who is laying it and is is possible to lay 2 like that in one day? I mix poultry spice with any veg in the afternoon and they all have access to mineral grit. They also get their own eggshells baked and put back in their food each day.