The size of the boxes is important to them as well. Heavy breeds like them about 12" across, smaller as the breed gets smaller. Some individual birds like different heights, so it's nice to have several options for them. Also, make sure other areas in the coop don't look as nesty and nice as those boxes. Make sure they can jump up into them. I store a couple of hay bales under mine. It took awhile for our banty hens to choose the cozy nesting boxes up top, and I had to consistently remove the spots they chose in the coop. When they chose to lay outside of the coop, I shut the flock inside for 3 days.
All it takes is one enterprising hen to discover the boxes, and you'll have success. If one hen thinks it's peachy, all the others will fall in line. Pick one up and set her in to check it out. If you have a rooster, do the same, and you might find him start cooing and clucking for all the world like he has an egg to lay. This is one of his rooster jobs, scoping out good laying sites for the girls.
I would chuck that carpet ASAP. It is a hiding spot for all kinds of mites and other stuff you do not want to encourage.
If you are using straw in the rest of the coop, they could be happy laying there instead of the boxes. Try removing the straw from the rest of the coop entirely for a few days until they get the point. Once they discover the nesting boxes, you can add the straw back in.