I had that problem when mine first started laying but I have always fed them chicken food, mine like the laying pellets, and they free range in the afternoon. I did add some oyster shell as well and then once they started laying on a regular basis the soft shelled eggs stopped until today! This morning when I went to let the girls out here comes my EE (Millie) with something in her mouth, I looked and realized it was a soft shell, I grabbed it out of her mouth and then looked in the coop and there was another soft sell, there were 2 eggs (I could tell because slightly different colors), So I will be making sure that they get oyster shells again!
Oh yeah, I forgot, I would feed them back the egg shells, I would heat them in the microwave for 30-40 seconds to dry them out and then crush them up so they wouldn't realize they were the egg shells. They loved that! Good luck