Some girls just seem to need more or maybe have trouble metabolising it. I have one like this.
I leave oyster shell (known here as shell grit) out all the time and they will each take a little when they feel they need it. I also have a Calcium liquid supplement called Calcivet. This also has Vitamin D3 added - which is essential if the hen is to absorb the calcium effectively. I mix this with water and add to their wet mash about once a week. You can just direct dose the one hen who needs it, if you prefer.
I still get problems from time to time - in fact, mostly a completely shell-less egg laid off the roost, like she doesn't even recognise that an egg is coming. I asked for more advice and a BYCer recommended liquid trace elements and amino acids. I buy this here in Australia as TraceMin, but you could probably source it online from a vet supplier or poultry online store. This suggests adding three times per week. Again, I just add it about once a week.
I have had no problems since doing this, so maybe start with shell grit and liquid calcium, then add trace elements if it still continues.
Good luck!
