I got a couple of these when my girls first starting laying, as well as few "rubber eggs". They were on layer food and had oyster shell, grit, and I saved and crushed eggs shells for them, so we were doing all of the right things. Sometimes it just takes the entire egg factory a while to sync. I was having trouble with one pullet in particular giving rubber and shell-less eggs, and that was in July. It's now September and she lays the nicest eggs - usually large to extra large in size, great color, and she's surprised me with one egg that weighed in at 112 grams and another that was 123 grams. Those eggs contained two complete eggs - two outer whites, two inner whites and two yolks, all within the single shell. That might sound great but it tells me that her timing is still just a little off.
As long as your girls are getting a good diet the factories should sync up soon.