Think of curses as as a stream of dark energy. In order for it to manifest, it has to solidify itself as a spell. However, the process of solidification is quite intricate. It depends on various factors such as intent of the person casting the curse, resistance of target, process involved in incribing the curse etc. Also, proper manifestation of spell requires a step by step ritual to be followed by the person casting the curse. Moreover, the curse intensifies if more people wish for that to happen.
When the person curses someone, a stream of negative energy releases from its body creating a void within him. Greater the curse, greater is the void created within. However, this void makes the spellcaster more susceptible to receive curses from other sources. Hence, the natural healing mechanism of the body kicks in, absorbing the curse back and filling the void. The rate at which the curse is absorbed depends upon the intent of spellcaster. More malicious the intent, greater the rate of spell solidification, lesser the rate of reabsorption.
So the daily curses like "I hate you Jim I hope you trip" seemingly have no effect as the spellcaster does not have malicious intent towards the target. Curses like these, spoken at the spur of the moment, are quickly absorbed negating it's effect. But curses written in stone or spoken a different way do have an effect as they are cast on purpose. Spellcaster has a malicious intent towards the victim and he wishes the curse to manifest and this accelerates solidification of curse into spell, thereby reducing its reabsorption. If the spellcaster's intent is strong enough, it also casts a rudimentary protection over the void created within to reduce its natural healing.
Yet, the intent itself is not enough for casting more complex curses. Normal folks with minimal spellcasting knowledge can make their curses effective with right intent, but it requires a real wizard/witch to cast highly devastating curses. Greater curses create greater voids thereby increasing the rate of curse reabsorption. A wizard/witch knows the exact ritual that seals the void created before the healing mechanism kicks in, thereby stabilizing the curse. The curses cast by specialists are more stable as they seal the void completely. As long as they can maintain the seal, the curse is effective. Greater the wizard/witch, greater the amount of void they can seal, greater the curses they can manifest. However, they have to be careful in estimating the amount of void they can hold. If they are not able to hold the void, the curse retaliates and brings their own destruction.
To overcome this problem, many witches and wizards work together to hold the void together. Many of them also exploit the intent of others to strengthen the curse. For example, for banning bishop Wulfric from building a cathedral over the ruins of the last one, the wizard exploits people's grudges against bishop Wulfric and their intent against building the cathedral and links them with the curse to strengthen it.
To carry out the curse of such magnitude specialists are required and someone would definitely ask for services of the wizard thereby making it valid. While a smaller level curse like forcing Otto to scream when he lies would be cast by a normal folk with hardly any malicious intent on a deeper level making it invalid.