Thick Membrane Eggs

[RIVERA69R]

My barred rocks and buff orphington started laying about 3 weeks ago now and from the start their eggs all had thick shells but in some of them the membrane under the shell is so durable i have to cut it to get it opened I dont use oyster shells at all here in florida theirs sand and shell all over but is this something that could change as they progress in egg production or are they just hardy birds