They're most likely taking dust baths in the dirt. This helps keep them cool during the heat and also rids them of mites/lice by moving the dirt through their feathers. It is really weird to watch a chicken dust bathe--they look like they're flailing around, I thought mine were having a seizure the first time I saw it.
Be sure that your nesting boxes are lower than your roosts, be sure they're big enough so a hen can sit inside comfortably but also provide some measure of privacy/darkness. The hen would rather feel secluded than exposed. Straw is nice but other materials work well, too. I put two inches of sand in the bottom to aid clean-up in case there's a lot of poop or a broken egg. Then, I have a few inches of straw on top of that. Makes for a well-cushioned box.
When it gets close to 20 weeks start looking carefully for eggs. I found a nice little nest of three eggs on the floor of the coop because my NHR started laying before I thought she would and I'd had the nesting boxes blocked off because they'd been sleeping in there. I simply moved those eggs up into the nesting box, thoroughly disturbed the area where she'd been laying, and she transitioned up to the nesting box without any problem. All the other hens followed her suit when they began to lay shortly thereafter.