The problem is very simple. Stock sets if there is enough natural gelatine from the carcase, so make sure you put all the bones, wing tips, skin and cartilage from the breast bone in If you use too much water it gets diluted and won't set, so after you have finished, boil it down until it reduces by half and it should set. The other natural alternative is to cook a pig's trotter or even half one (the butcher will split it for you) in with the carcase and you will soon have a jelly set. Temperature is the other factor - stick it in the fridge or even the back garden in a plastic bag and it should set.
I usually get carcases from the butcher - ask him to save what's left after he has taken the chicken breasts and legs off. I never get one which doens't set!