Curses are exacting, legal arrangements of the spiritual world. Just like human contracts contain fine print and legally crafted language, satanic curses are filled with minutiae that required detailed voiding.

Satan is always looking for legal right and authority to torment and attack people. Whatever he has the right to do, the devil will do.

Humans have will. In an exorcism with a human, you are dealing with the human will, and whether that will is sufficiently resolved in terms of what allowed it to be manipulated. The will must have done something to surrender to the presence of the demon. You have to resurrect the moral authority of the person's will.