I'm very new to raising chickens. My hens are approaching 22 weeks. Two hens are doing great (RIR and Ameracauna) and each are giving me one egg per day, even through heat waves. My white wyandotte isn't doing as well. She was the first to lay, at 18 weeks. She layed one egg per day for about a week, spent a few days just landaying yokes, then a few days laying eggs with very soft shells. I gave her yogurt mixed with bananas and we were back to big beautiful eggs again. For a few days. Then 5 days ago she stopped laying. She seems happy, runs around with the other chickens and eats/drinks plenty. All my girls are fed layer crumbles. I provide them with oyster shells too. What can the problem be? We've had a very hot summer, many days of 90 degrees+ plus humidity. Could it be the weather or is there something else i should be doing to help her?