SLW why won't she lay in the nest boxes

[Darklingstorm]

I have 9 chickens all laying, 3 of which are SLW. I have a coop with 6 nest boxes with fake eggs in them. I also have a pair of ducks and she should start laying soon too. However the darn SLW chickens will not lay in the coop nest boxes but prefer the duck hut! This fall I'm going to be separating the ducks into their own place, no access for the chickens.
How do I get them to lay in the nest boxes in the coop? Due to the heat, I can't leave the chickens locked inside the coop all day, but I do leave them in there till 9-10am.