Hen laying in an odd spot...what to do?

Ok, this is a very hard question for me to figure out on my own so I need the help of BYC's chicken experts to help me.

I have around 18 years experience with raising, breeding and keeping chickens. I have had all types of chickens and love them dearly. Today I have six Red Sex-Link hens that are in my "laying" flock. Though they are confined all the time (to protect my vegetable garden) they still produce five eggs a day. This is a good sign because unhappy hens do not lay an egg. They are not flighty or aggressive and are somewhat friendly. They are kept with a above-average size Hew Hampshire red rooster (12 lbs). I need fertile eggs for incubation so that my girls (now three years old) can retire next year and the offspring can take there place. My retired girls will then be moved to a new coop until they pass on. (I simply don't have the heart to eat them)

Here is the problem:

One of my girls is laying her eggs on the floor of the coop. I noticed it about four days ago and never thought about it twice. I became concerned but not overly. I simply expected it was not done one purpose. After four days of eggs on the floor I am wondering, how to correct this or if there is a problem with the coop or nests that I am not aware of. I know that fake eggs work to encourage them to use the nest but I use golf balls and it works. (all the nests have one)

Remember these all live together and I have no idea which hen is doing this. They have been in this coop around three months and all appear to be happy. I picked them up one at a time and listened for respiration troubles and none sounded bad. None had mites or any other symptom of disease. I looked on the floor and found no signs of loose stools. They were dusted around two weeks ago but only for maintenance not because any of them had mites. The only resent possible stress they could have had was when I re-homed three hens for retirement but nothing else.

Simply put....I am very lost. There is some information online but it all pertains to the hens health or the height or quality of the nests. The nest are below the roost and are all well-bedded with pine wood shavings. The nests are around 2 - 2.5 feet off the ground and are lower than the roost. None of them roost on the nests but sometime the lower ranking hens will perch there during the day if is hot out. The nests are twelve by twelve and are VERY clean. The six hens have access to four different nests that are very far apart. I know that hens prefer to lay in place that is dark but these hens have been using these nests (or exact copies in a different coop) for a very long time and I've never had this problem. Each nest gets one egg a day except one that gets too. The nest with two eggs is no different than the others and is set up exactly the same.

ANY IDEAS? All advice appreciated!

Pictures of the coop: The hen on the nest simply refused to move until I left. (I have to check if she roosts on the nest and correct her)

Look what a big pretty rooster:



The entire flock:



The coop:



The tin around the nest prevents a draft and the roof does not leak:

 

He is quite the Romeo with them. They are older than he is and their feathers are "fuzzing" at times due to his weight, but he is pretty good. He is the only Heavy-Breed rooster I have ever seen fight with a rooster (by accident) and kill the other rooster. The odd part of this is that the other rooster was an American Gamefowl! He is a very gentle guy with people though. I have had some people question his breed thinking he was a Production Red but I assure you he is simply a non-show quality EXTRA LARGE New Hampshire Rooster.


PS
The breed combination of him and the hens produces a egg-layer type hen that weigh 10 lbs and still lay six eggs a week like there mom's!


Tim