I have a couple of questions
How long are you cooking the stock? I believe it something like 8 hours.
Did you break one of the bones to check if it was brittle. The brittle bone is supposed to be the indicator that all of the connective goodness is out of the bone and in your stock.
Lastly, when you chilled it and it wasn't thick and gelly did you put it back on the stove and cook it some more. This is supposed to thicken it up.
I haven't made Alton's chicken stock, but I have made stock before usually all it takes is a lot of simmering.