I have 5 chickens at 25 weeks of age. White Leghorn, Silver-laced Wyandotte, RIR, NHR, and Buff Orpington (Lila). It has taken a while to figure this out, but it is the Buff Orpington struggling to produce an egg with a viable shell. She is not showing any signs of illness. All 5 free range in a large enclosed garden area with a lot of vegetation. They are on Hiland Natural layer feed. They have oyster shell available all the time. Carefully regulated treats include chicken scratch and meal worms. Lila was the last to start laying eggs, in the last 2-3 weeks, she has produced 6 soft-shelled or rubber eggs. The other 4 girls are doing great. I am at a loss at this point.