Could be any number of reasons.  How old are your hens?  My hens have a favorite nesting box, and will wait and cackle at each other if they want to lay and another hen is in that box.  I have 12 hens and six nesting boxes and yet they only want to lay in one or two of them.  If they're young - it may be that they just haven't figured it out yet.  Lastly I'd say that if the nesting boxes aren't clean or have a nesting material your girls don't like, they'll find another place to deposit their eggs.  Every once in awhile I'll find an egg in an odd place - but more often than not the girls now lay where they're supposed to.  Good luck.