Why is only of my hens not laying her eggs in the nesting box?

[brandi21]

I have 6 hens. Two are white leg horns and they are great layers! I get 2 eggs everyday from them and they have never skipped a day and their shells are absolutely perfect! I also have four rhode island reds but only 2 of them are laying. The one lays in the nesting box everyday with no problem and has a great shell! However, one of the others lays on one of the roosts that is in their coop. Luckily I have a deck under so instead of falling on the coop floor, it falls on the deck which is easy clean up but unfortunately it breaks and they eat it. She only lays about every other day. Today was her 4th egg and I finally caught them in the act of eating it and to me it looked like a very soft shell. I scrapped it out to show my boyfriend. I am concerned as to why she is NOT laying in the nesting box and why she is the only one that has a soft shell. Also they do not touch/eat any of the eggs that are in the nesting boxes. Only the one by the roost. Any ideas or suggestions??