Many hens stop laying when it's too hot... Many hens stop laying when it's too cold. Almost all hens will stop laying if they don't feel good or have worms or mites. (Have you checked for mites around her vent (butt/egg canal), under her "armpits"). Have you wormed, when, with what? Is your entire flock 6 months old.. are all your pullets or all your chickens young?
It seems my Easter Eggers stop laying at the drop of a hat... I have one that has not layed in two or three months, but she's fine otherwise. I am worming now and will see if she starts up again... if not.. well.. I love my chickens, but keep them for eggs and I will have to cull her if she doesn't start laying again, feed is just too expensive.
Some of my young hens, the 5 to 6 month old ones started laying, had some soft eggs, some small but "regular" eggs, then stopped laying, started again after a couple of weeks, stopped a few days, etc... so.. there can be stops and starts with young ones. Just as long as she feels good and is not sitting on a nest and not laying... if that is happening she may be eggbound. Check her vent for any abnormalities, rub her stomach under and around the vent, see if you feel an egg inside (careful not to break it).
Use the search here for lots of info on any symptom or subject... like worming with Valbazen. Laying Pellets. Eggbound. Stopped laying. Etc. I assume you have looked up Molted Japanese to see their breed traits, what kind of weather they tolerage best, what type and kind of eggs they lay, whether they are good or poor layers, Etc.
Good luck.
