You have some sneaky hens! I finally broke mine of laying everywhere. I literally followed them for an entire day. I took their eggs and put some of each in the nest boxes, tore up all of the nests in the yard, cut ferns/bushes to discourage them from making more and gave them a stern talkin to. They have been laying in their boxes since!
edited to add quote
The stern talking to didn't work. Not only are they sneaky but stubborn! I'm going to have to figure out a way to block off the shed from the girls since clipping their wings did nothing to deter them from going in the front yard!