There are a lot of things that could be going on here. Maybe they are molting. Maybe they are eating the eggs. Maybe something else (dog, raccoon, snake, bird, possum, human) is stealing the eggs. Maybe they are laying the eggs somewhere where you can't find them. Maybe they aren't getting enough protein in their diet. Maybe their eggs have paper-thin shells due to low calcium and they are getting trampled and mixed into the bedding. Maybe they are laying fewer eggs because of the shorter daylight hours. Maybe they are sick. Maybe it's a combination of several of the above.
I would take a good look at all of these to rule them out. If they free range, consider locking them in for a few days or a week so they can't hide their eggs. Check for spots where an egg thief might get in. Examine them closely for pin feathers, mites, worms, general health and appearance. Look at their eyes, feathers, feet, and combs. Feed them good, high-protein layer ration. Give them access to oyster shell or other supplemental calcium.
Sorry to be so broad, but without knowing what you've ruled out, it's hard to be specific. There is also a whole thread dedicated to the question of why hens might not lay.