My girls still don't get the roost and nest box thing -- they can roost on our glider and poop all over it, but hop up on the roost in their coop? No way!
WHen mine started laying, they did so in the corner of the run in a big pile of stray. Once the weather turned, though, they started laying in the corner of their coop. I gave up on them using the raised nesting box built into the coop and built a new nest box. I put it in the corner where they had been laying. I had to catch one of them getting ready to lay and put her in the nest box to convince her that it was a good idea, but now they lay in the box.
Sooooo... if your boxes are portable, maybe try putting one where they usually lay and then moving it gradually to where you want it to be permanently?