Shell quality deterioration -- can it be turned around?

One of my 2 yr old RSLs has started laying eggs with paper thin shells. When I first see them in the nest box (from the wrong side of the poultry wire) they look great, but by the time I get in to collect them they've been broken and I can see there was virtually no substance to the shell. This has been fairly consistent over the past 2-3 weeks -- she isn't laying every day anymore, but every time she does lay the egg gets broken.

I'm feeding 15% layer mash free choice, plus a variety of supplements to choose from including oyster shell, fish meal, and kelp meal. Like everyone else, she free ranges from before dawn till whenever she decides to go to bed. Is there another supplement I can add that might help bring her shells back up to par or is she just showing signs of having laid all the eggs her body wants to produce?