I suggest that you add a second nest box and put a couple of golf balls in it to give the hens a hint. If the hen sleeping in the nest box also seems rather grumpy, she may be broody and hogging the current box. Hens can only make a certain amount of egg shell material so your over-achieving layer may be laying thin (and maybe pale) eggs due to exhaustion. I know of nothing you can do to help. Also be sure your roosts are higher than the nest boxes.
