I had a hen that would walk straight past the pellet dish first thing and fill up on grass.
I stopped the free ranging for the first half hour so if they were hungry (always) it was pellets or nothing.
That way you know that with the pre roost fill up with pellets they have a good amount of calcium.
Always had a dish of oyster shell, NEVER in years saw one eat any of it !
Sometimes no amount of added calcium will stop thin shells as the problem is with how the hen is forming the egg , no suprise as it is a small miracle in itself !