Pecking order problems... I suppose you've got a heavy-breed such as Rhode islands, or australorps?
Well, there's a few things you can try, number one is seperation (however another hen will become low man on the totem pole, and may be brutalised for the sake of her attacker's status) This can work quite well, especially if you want to add to your flock, as you can pick up some pullets to add to a second coup and yard that will invariably become subordinate to the formerly low hen on the pole.
You can try clipping the agressor's beaks, I haven't done that, because the aggressors are usually really good layers, and tend to keep the rest of your flock in line.
If you're not interested in clipping, try culling! The weather *is* changing, and Chicken & dumplings is quite a tasty dish on those cold fall evenings.
I prefer to seperate, and add to the flock. You'll be able to try out different combinations of layer & grains & forage to find the best mix for laying, all the while maintaining a control group (your original flock) that will keep you fryin' eggs until you've prooved a different regimen to be better.
Dad
"A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently and die gallantly. Specialization is for insects." Robert Heinlein.