one of my black rocks does this sometimes - we either get rubbery 'shell' or a very wrinkled paper thin one. Like yours, ours have oyster shell, layers pellets and free range.
One thing I would say (having found out the hard way!!) is get rid of all the soft shell eggs asap, my sussex has developed a taste for eggs since BR laid some softies and now I have to collect the eggs very quick for risk of losing them to her (although our ceramic eggs have put a bit of a stop to it)