You can give an emergency dose of 1 crushed Tums mixed with plain active-culture yogurt. Plus, if you have it, add 1/2 teaspoon of cod liver oil (for the vitamin D). Getting enough calcium is crucial for hens who are laying lots of eggs - like this time of year. Feeding crushed egg shells is not the greatest supplement as the calcium in egg shells is not as bio-available for hens as is the calcium carbonate of crushed oyster shells. Tums is not calcium carbonate, but it is easier to do an emergency dose with Tums than with oyster shell. So, don't depend on feeding them Tums; it's just an emergency measure. They need oyster shell, long term.
Do NOT stop offering crushed oyster shell. But don't put it into their feed. Offer it in a free-choice style dish, always available, so that as a hen needs it, she can help herself to what she needs, when she needs it. And she will, if you make sure your oyster shell dish is always topped off and full. I learned this lesson recently. Not all hens will continue eating oyster shell when it gets low in the dish and powdery. They'll skip it, and then their eggs will become rubbery and difficult for them to push a soft egg out of the body.
I just cleaned & dried out a large tuna fish can and nailed it, at the height of their backs, to the side of their run (under a roof so it stays dry). I fill it with oyster shell all the time & keep it full so they will feel like eating it. And, even though they have a very high-quality feed, my hens eat a LOT of oyster shell. I see them eating it all the time.
