Why is my stock jelly-like when cold but watery when hot? I made a chili with home-made stock that appeared to have plenty of gelatin in it as the chili wobbled like a jelly when defrosted but when I subsequently heated it the sauce was very watery.

Is this a case of simply not reducing my sauce enough when I originally made it (despite its jellyness) or is something else going on?
The properties of gelatin are that it gels when it cools and turns liquid above 90F (32C). There's nothing wrong, it's supposed to do that. The melted gelatin has a silky mouth feel which many consider desirable.