Calcium overdose can cause kidney failure and/or gout. Unfortunately it is something that will not become apparent until the damage is done and will build up over a long period of time before it gets to a critical point. Some birds will tolerate an overdose better than others. It may be that there is a high level of calcium in your soil or they are getting what they need from other sources.
It is less of a concern if you are not feeding a layer feed but that was not mentioned in your other post and other people reading this might think that adding it to feed is necessary and overdose their birds as a result.
Hens should know when they need extra calcium just like we know when we are thirsty. It is an instinctive response. Animals are much more attuned to their body's needs than we humans are. Birds know when they need grit and calcium and which vegetation is safe to eat and which isn't. I have horses that will stop when we are out riding and insist on eating mud in a certain part of the woods... there is obviously some mineral in that mud in that particular spot in the woods that has something in it that they feel they need because it is always the same spot even if we haven't been there for months or even years. Two of my mares dug up a huge boulder that was just under the surface of their paddock and would stand for ages licking it. The geldings never bothered it so it was probably something to do with the female hormonal system, but they knew that that was what they needed.
A little bit of oyster shell goes a long way so it may have been that your expectation of their consumption was too high or they were getting enough calcium from another source be it in animal, vegetable or mineral form.... this is probably more relevant if they free range and have access to a variety of terrain and vegetation.
If you are concerned that they are ignoring the crushed oyster shells or egg shells, scattering them on the ground will draw their attention to them and perhaps encourage them but still give them the option to take it or leave it, whereas putting it in the feeder can make it compulsory/unavoidable.