"Usually" most hens WANT to lay in a nest box of some sort because it gives them the cover, secluded privacy that they like. Open "nests" in the dirt are typical of Beaulieu's that they like to bath in, not necessarily lay in. As previously mentioned put some golf balls in your nests and make sure the nest box (of whatever make and design) are easily accessible. Be watchful as 20 weeks is fairly typical for the beginning of laying for most breeds. You can research also to find out even more helpful hints on free ranging your chickens.
Wish you the best!