I'm no expert on this, just a fellow student of the art, sharing notes and experience.
How long it cooks for, the water content, the amount of collagen -- all of the above. Collagen converts to gelatin best around 180° or somewhere around there, basically a very low simmer. And it does take time, it doesn't happen instantly.
Stocks are based more on bone than meat (as opposed to broths), and so they get gelatin from the collagen in bones and marrow but also especially from cartilage, which is loaded with it. So joints are good. Skin also has a lot of collagen. When I'm making chicken stock I often throw skin, necks, and feet in with the bones in order to maximize gelatin, because they're all great sources of collagen.
Gelatin is just one factor in stock-making, and it sounds like on that count you're doing great!