Dense egg membrane

My 4 pullets (1 Plymouth Rock, 1 Ameracauna, 2 Australorpes) lay eggs with very dense membranes.  It's nearly impossible to get a cleanly peeled hard boiled egg; cracking an egg into a frying pan or bowl takes a very firm action.  Could this dense membrane be caused what they're eating?  They feast on Purina Layer crumbles, kitchen scraps, plus whatever they get free ranging for about an hour or so most days.  I recently have starting adding some oyster shell to their layer crumbles.   Any wisdom you can share with me on this topic is greatly appreciated.