the larger end of my hen's eggs are always very thin...

[Chloebird]

I am not sure which hen is laying those eggs, but almost everyday I find an egg that is shaded darker on the larger end and if I touch that spot I can easily break right through. It just seems odd that it is only thin on that one end! I have been giving oyster shells free choice, but they don't seem to eat it, so for the past few weeks I have been mixing some right in their feed. Any thoughts??