Hi
I've only ever had this problem with 1 chicken. Mine are all Rhode Rocks except this particular chicken who was a Rhode Island Red.
At first i thought it was because they were queing to use the nest box. Although we have more than enough nest boxes they all insist on laying in the same box.
This hen in particular was at the very bottom on the pecking order and was always last to lay. She always laid hers under a bush rather than the nest box and did this from when she first started laying.
I now wonder if it was because the nest box was too small for her. I only wonder this because my Rhode Rocks are much smaller than she was and they seem to fill it (the nest boxes on the coop i am using at the moment are smaller than the nest boxes in my old coop which is standing empty)
I dont have that chicken any more otherwise i would see if she would have prefered to lay in there instead. Of course that may not have been the problem at all , maybe she just prefered to lay under a bush or felt safer there.
I still sometimes find the odd egg under a perch instead of in the box (yesterday) but this was because they were queuing and i found 2 hens trying to lay in the same box at the same time and had to move one on, probably hers under the perch.
If all of your chickens are the same size and the others are laying in there alright, i shouldnt think the size of the box would make a difference.