I have been making my own stock for several years. I make it EXACTLY the same way every time. Sometimes it has a "gel" consistency,(which is awesome), and sometimes it is just a liquid.(still delicious.). What is the determining factor that dictates whether it turnes into a gel or not?
The difference is in how much collagen you can extract and turn into gelatin from the meat & bones.
If you can, split or crack the bones before you start, then place them into the pot, cover with relatively room temperature water, and then slowly warm it up. If it reaches a boil, lower the heat.
I've been told that the reason for this is that if you put the bones into hot water, the marrow swells up, preventing the water from getting deeper into the bones. I've never tried it the other way, so can't confirm this.
If you have any cartilage, add that with the bones. You can also roast the bones first for more flavor.
Meat selection also plays a role -- lean meats tend to have less collagen, while the tougher cuts tend to have more. For beef, try to find oxtail or shanks; for pigs, split hooves, ears; for chicken feet, necks, and wings.
... and if it still comes out too thin ... bloom some gelatin in room temperature water, and then stir it in.