new texture on shells

We have 25 ISA Browns that are very good egg producers. We are located in Michigan and it is now winter. They have an enclosed coop, a light on a timer, layer mash, crushed oyster shells and an adequate water supply. They seem to have a good routine and don't appreciate having it interrupted. They are very vocal and let us know. We get between 22 and 24 eggs daily so we are ok with that. What we are noticing now is that the eggs no longer have that smooth shiny finish on the shells. They have a rough texture and are not the same deep brown color they were in the summer. Sometimes one or two of them are actually white. We have tomatoes, zucchini, etc in the freezer to give them as supplemental feed. Also give them leftovers from the fridge. We try to follow the food guideline posted by others on this site. No potato peels, etc. What is the problem with the egg shell texture? Are they missing something in their environment or feed?