It sounds like you are doing everything you should---- so therfore your chickens are Incorrigible, they are willing to give you eggs, but they want to aggrivate you as much as possible. Isn't that just like a pet.
so... try a real or fake egg, and maybe a nest box that is either a little bigger or a little more open--and as quiet, private and shaded as possible.....
My setup is an Eglu Go, inside there is more of a nesting area---than a true box with sides. My 3 always lay in there...sometimes when I look in at them they seem almost too big for the space they are laying in---but it seems to be working out fine..... Chickens can be fussy about that.
Maybe as a last resort--- expect them to lay outside and work a nesting box arrangement into the area beside the compost? I had/have thought about making a pen of PVC pipe for mine, and then I keep thinking I have to wait until the last one has laid, before I turn them in---or I would need some nesting box arrangement....
Post back if you find the key to the dilemma!