Why is my stock from stewing hen less gelatinous from that made from a 4-lb broiler? I cooked it twice as long to extract flavor, but no gelatin.

It could be related to the age of the bird. Gelatin comes from the breakdown of connective tissue, and cartilage is the best source. Younger animals have a higher percentage of cartilage in their bodies (as animals grow older some cartilage hardens into bone, reducing our ability to extract cartilage). Knuckle bones, neck bones, and shank bones also yield higher amounts of gelatin. It can still be a tasty broth, but it won't have as much gelatin. Fine for soup, not so good for aspic.

No, the old hen usually provides the best flavored stock. I think the problem lies in the long cooking time. Gelatin is fairly heat resistant, but long exposure to heat will cause it to break down. I think that's probably your problem here. I know old recipes will say to simmer for 8 hours or so, but the simmer heat needs to be kept low enough that you really don't see any bubble formation. If it gets too hot for too long, the gelatin strands break down and can't reform to hold on to the water molecules as it cools. Your broth will likely taste wonderful, but if you want the unctuous mouth feel, you will probably need to add some supplemental gelatin, either in powder or sheet form.

Yes, I included wings.

Did you include the wings? A lot of collagen/gelatin in the wings.