Brian Monetti, I spent several years working in home improvement and construction
First, the breaker is still being tripped. Imagine a shorted out toaster oven that trips the breaker. If you don't unplug the toaster or fix it, and you try to reset the breaker, it will not reset. The short that is tripping your breaker might not even be visible to you. Sometimes it could be something as obvious as a burnt out toaster oven, but other times it might be a frayed wire in a wall!
The second possibility is that the breaker itself is broken. These things are pretty robust though, and aside from really old ones they rarely break. But it does happen!
Either way, you should really get a trained electrician to find the problem/solution. Finding a hidden short can be a real pain if you don't know what you are looking for, and potentially dangerous. Also, changing a breaker has the potential to be life threatening if you do not know exactly how to do it. Iv spent years working in home construction, and even I would make a phone call to an electrician before going further here because of the potential problems that could arise if I do something wrong with electricity.