There are a couple of things that could be going on.
1. She is broody. Hens that are broody don't lay.
2. She is getting over a moult.
3. She is too young/too old to lay
4. She is laying, but you just happened to watch on a day when she didn't lay an egg
#1--When you say she "takes care of the other hens' eggs," what do you mean? A broody hen will stay on the nest almost the entire day. She will be very fluffed and very flattened on the nest, as if she's trying to make herself cover as much area as possible. She will often growl or peck you if you put your hand in the nest. If you pull her off the nest, then she will often stay in that flat position on the ground for a few seconds before she walks off, and if you come back 20 minutes later, she'll be back on the nest. If this describes your hen, search BYC for "breaking a broody."
#2--a hen can have a light moult and you will hardly see it. Check her feathers, and see if she has any bald spots or any new pin feathers growing in. If she's moulting, you'll have to wait until she's done. No real way to hurry that.
#3--I doubt she's too young or old, since you say she's crouching, which is a sign of sexual readiness. If she's too young, she will have a smaller, paler comb than the rest. For an aged hen, check her feet and legs relative to the others. Older hens have much scalier, rougher looking feet and legs.
#4--Very few hens in the world lay eggs every single day. Even for sex links, 5-6 eggs per week is more common. You might just have chosen to watch on a day she didn't lay.
It's not easy to tell if a hen is laying, but there are signs. Over the vent are 2 pelvic bones. On a large hen, 3 fingers should be able to fit between those bones on a laying hen. If you can only fit 2 fingers, the hen is not laying. Her comb should be red and plump. Her vent should be red and moist. If she's looking "dried up" then she's not laying for some reason--see # 1 and 2 above.
