Look for another cause. It's not the garlic.
How much protein are they getting? They may be a little low. Are they getting crushed oyster shell, for the calcium?
They may be in early stages of molting, or there may be a nest you haven't found, even though you looked really well. Hens are sneaky.
Do they have parasites? Mites, or worms? The garlic would probably have killed any worms, but have you checked for mites?
So from 18 hens, you've only gotten 6 or 7 eggs a day? How old are they? What breed? Hens in good condition, unless they're too young or too old, should be giving you something like 12-18 eggs a day. Except when brooding or molting.
It shouldn't be length of day, it's only August. If it was mid-winter, then maybe. My heavy breed hens lay all winter. Without added light.
A hen that has gone broody does not stop just because there are no eggs. Or in this case, no rock. They will go through the broody cycle, some will quit after about three weeks and no chicks, others will brood forever if you let them, (or until they die) until they either hatch a chick, get a day old chick or two slipped under them to adopt, or you break the broody cycle, by putting them where they have no place to nest. Like a bare dirt pen with food, water, and a roost, but no boxes or nests, or anything of the sort.
