It works the other way too. Mine usually get soaked feed; they think dry feed is a treat.Are you sure they are eating it all? Check the floor around the dish, under the bedding, to see if they are knocking it out of the dish.
It's common for chickens to knock food out of a dish while trying to find the perfect piece, so yours might be doing that with their grit.
It is normal for chickens to eat small amounts of food all day long. As long as they eat the right amount in the course of the day, they will be fine. Most chickens are good at eating the right amount of food, as long as they don't get too many treats.
If you want a "treat" that many chickens love, try adding water to a little of their normal food. Apparently it tastes much better wet