Funny thing about calcium.....it works until it doesn’t.
First off, don’t panic and start throwing the kitchen sink at her. If all of the eggs you were getting from the whole flock had poor quality shells, you could look at diet because the entire flock would be affected - some more and some less than others. But you suspect that it’s only one hen. So that narrows things down a bit.
Look at breed or variety. Is she strictly a production breed? The only chicken I ever had that laid soft shelled or shell-less eggs was Beatrice, a Red Sex Link. Heavy producers are usually - and I stress ‘usually’ because there are always exceptions - usually only really good for a couple of years, then egg quality and quantity can begin to wane.
As
@EggSighted4Life said, heat can absolutely affect egg shell quality.
And now we come to calcium, that pesky but oh-so-important component in quality egg shells. We can provide containers of it all over the place, but what we can’t do is stand out there and monitor their intake. We don’t know for sure if she’s actually eating it or if she’s even get enough of it when she does. Remember that soft shelled or shell-less eggs can also occur even though a flock is being fed layer containing a good percentage of calcium, and oyster shell on the side.
Just as there are people who can eat all of the Vitamin B12 rich foods known to man but their bodies can’t properly absorb and utilize it, some chickens can’t properly absorb and use the calcium they do take in. So if that’s her issue, once her body used up it’s natural stores of calcium she could eat oyster shell as her main food and it wouldn’t make a difference.
Can we go back to my Red Sex Link, Beatrice? She started laying rubber eggs rather consistently. At that time my flock was still on layer feed with oyster shell on the side? What was going on? She wasn’t a new layer, so it wasn’t a matter of those being her first eggs while her system got in sync. She wasn’t just starting to come out of a molting period. Temps were still moderate in late May. We decided to cull her, but she saved us the chore of doing that. We did a clumsy, amateur necropsy and her reproductive system was a mess with tumors scattered throughout. Now, DON’T panic and assume the worst. My point is not that that is what’s happening with your hen. I’m merely throwing out other possibilities.
I know a lot of people add ACV to the water in the summer to cut down on algae growth and for other reasons. I did too, in the beginning. What I’d read made it seem essential for healthy chickens. But then I ran out and kept forgetting to get more. Can you guess what happened to my flock? Absolutely nothing. They were healthy with no internal parasite problems. Their feathers were glossy and they were active. And I honestly think their shell quality increased a bit. I don’t know if the change in Ph would have made a difference for Beatrice - she was gone by then. So I can’t and won’t say that stopping the ACV improved her shell quality. But I will state that, based on my experience, ACV may change the Ph in their bodies enough to affect the shells.
As easy as they are to take care of (I think, anyway), they do have rather complex reproductive systems and many things can have an adverse affect on that. I have fed either All-Flock or Grower consistently now for years, providing oyster shell on the side for the layers. Raising chicks out with the adults means that my flock is usually a mix of roosters, assorted ages of chicks, layers, and freeloaders. I don’t have the time or the money to sweat the gender/age/production stuff and can’t keep them out of each other’s food anyway. And I haven’t had any problems with the health of my flock or the egg quality.
Finally, her timing may just be a bit off and she’s moving the eggs in her system through too fast. Not enough time for the shell gland to work also results in soft shelled eggs. If that’s the case, she should get back in sync by herself without any other help.
So there. Now I’ve either helped you or confused you beyond belief! I just felt it was essential to point out that you weren’t doing anything “wrong”, and that issue isn’t always solved with more calcium.
