Strange Eggs...Any Ideas Why?

[chixelandpixel]

Lately my Uncle's Chickens have been laying eggs that wont crack. The inner membrane of the egg is so tough that even if I whack it on the counter edge it just cracks the outer hard shell but I can't get the yolk out to use as ingredients. Anybody out there have any clue whats causing this. We collect the eggs every day. It isnt old eggs.

Thanks to anyone who can can solve my problem.