Silkie laying soft shelled eggs

Any advice on Silkie layng soft shelled eggs. Have beefed up their grit. Have added organic yogurt with probiotics mixed with warm outmeal. Occasionly mix with wet cat food thought this might add more protien. She is approx.1.5 yrs. She's hatched out lots of healthy chicks and never had this problem before. Any advice would be greatly appreciated. Thanks.

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Thanks for all the information. I have already done everything you have suggested. What do you think of adding a little Vit D to the H2O or perhaps on the beak?