Usually if you give them a cozy area to nest, they will lay there until they decide they have enough eggs, then they will start to brood them (Muscovies anyway). I have had some first timers that laid in the yard for the first week or so but then moved into the boxes in the house.
One of my girls started digging out a nest in the corner of the coop but it was under the roost so i moved her eggs to a box and changed the bedding and messed the nest up. She should eventually start laying in one of the boxes, sometimes it just takes them time..... I also suspect it's my newer girl, Hazel, and she is used to nesting on the floor of the coop because her old home didn't have nest boxes, just a shed full of straw for them to lay & brood in.