Since you talk about crushed maize, I'll assume you are not a close neighbor. What is called maize other places is called corn in Arkansas.
Crushed shells are not grit. They are too soft to grind up hard food. Since chickens don't have molars to grind food, they use pea sized and smaller bits of gravel and sand in their gizzard to grind up hard stuff. You can probably buy grit, but I have been known to gather some small gravel off my gravel road and driveway to give them. If your road is salted for ice in winter, I would not suggest that since the salt is bad for them. If they have access to the ground, they are probably getting enough grit.
Chickens need a balanced diet. If all they eat is layer feed, they are getting that, but if you give them too much of other things, their diet may be out of whack. If they roam outside and have unlimited access to seeds, green stuff, and creepy crawlies, they are probably doing OK, but if all they eat is what you feed them, they may not be getting enough of some nutrients or vitamins if they get too many treats and are not eating enough of their layer feed. I kind of suspect an unbalanced diet as a part of your problem. Treat the crushed maize especially but even the cabbage as candy for chickens. Limit how much they get of that so they will eat a more balanced diet. Crushed maize is long on energy but short on nutrients, protein, and vitamins.
You mention they are not eating many of their pellets. How long have you had those layer pellets? Have they gone off? With only four hens, a bag can last a long time. It needs to be stored in a fairly airtight container and kept dry. Don't let it get too hot, like storing it in direct sunlight.
Let me offer a couple of links. This one talks about soft or thin shelled eggs. Lack of calcium is not the only potential problem.
http://www.thepoultrysite.com/publi...ndbook/16/thinshelled-eggs-and-shellless-eggs
And I've found this one helpful on feeding chickens.
Oregon State - Feeding Chickens
http://extension.oregonstate.edu/catalog/html/pnw/pnw477/#anchor1132074
Good luck.