I would first start by searching your yard for a hidden clutch of eggs. Just because one of the hens lays her eggs in the designated egg spot does not mean the other one will. That being said, even though they live in the same place and eat the same food, the environment most certainly can be whats bothering one and not the other. Some people get stressed about loud music and others do not, some chickens like this food or that while others do not. Another thing to consider, and I'm just offering up suggestions since you didn't mention age or breed, is that the non laying hen may not be as old as the one that is laying and just isn't ready -and- even if they are the same age, if they are different breeds some breeds mature slower than others which means they start laying later. If none of those factors seem to apply to you and you're certain she should be laying but isn't- start by having an Easter Egg hunt. If that doesn't turn up anything compare the two hens vents. The vent of your laying hen will be much wider and looser then a hen who isn't laying. If the vent of the culprit is tight like an egg has never passed through it, it's probably because it hasn't. Good luck!