Continued Soft Eggs from a Silkie

I've had two Partridge Silkies for about 5 months now. They're roughly a year or so old, and about 1.5-2yrs. One of them had started laying an occasionally soft egg. I've added crushed oyster shell, and they have Purina layer pellets. They also get dried mealworms, and sometimes a little scratch. I have also been giving them Bragg's ACV, and periodically Layer boost vitamin powder in their water. I don't believe that they're under much (if any) stress, and their daily routine is pretty steady. I don't see any signs of illness, or unusual behavior. She looks healthy, and eats, drinks, and forages well and is sociable so my other three chickens. She hasn't laid a soft egg in several days, but did this afternoon. What else could be causing the continued soft eggs? Please, limit your responses to the issue at hand.