I realise this is a really old post but am bumping because I have the same problem. I have a 7-8 month old sussex that has just started laying (at least I think it's her) but this has been going on for well over a week now. I think I have found a total of 8 or 9 eggs - all under the roosts in the sleeping coop, 1 in the middle of the lawn.
How long will it take her to work out she should go to the nest box? She is lower in the pecking order - might she be too scared to use the nest box if the other girls are there? I have about six laying hens and three nest boxes.