Rebellions are as difficult as starting a religion. Unless the realm government is completely dead/silent and you have a good group of engaged players backing you to take control of the government and actually do something for a change. Or unless the ruler just did something highly unpopular amongst nobility (banning someone well-loved, for capricious reasons etc.) and you can properly capitalize (usually helped by longevity in realm). Or the ruler just won his own rebellion and you're attempting the loyalist counter-rebellion. Having the General or Capital Lord on your side is always a huge help; you can almost predict a rebellion's success by whether the general or capital lord is supporting the rebellion or not.
