Egg Problems

Im sorry I cant tell you for sure...but here is what I would do ACV in the water....will help them absorb the oyster shell protiene...I would try that. Are the layer pellets the same? No change in feed? Maybe someone with more knowledge can tell you more. Good luck . Hope all gets better!
edited for spelling:

 

Are you giving them the oyster shell free choice, or is it mixed in with their food? Sometimes letting them choose as much as they want can help. Something else that can help is Apple Cider Vinegar, the organic, unpasteurized kind with the vinegar mother in it. It can help the birds metabolize the calcium. Let's see... what else can I think of... are they getting out in the sunlight? Vitamin D helps them to metabolize calcium, too. And adding kelp meal might help with any nutritional deficiencies in minor nutrients.



Does anyone else have any ideas?

 

Just a guess but maybe worms? Might not hurt to take a sample to your vet & have them check it out.
That's all I can think of for now..

 

How old are your chickens? The shell quality reduces with age.
Try boosting their calcium intake. I crush up their eggshells and feed them back to them. It's easy to do if you find a strong polythene bag and tread on them!