In the gospels we often read about Jesus casting out demons. But there is one episode, where the disciples were unable to cast out a demon (Mark 9: 27-28).
27 And when he was come into the house, his disciples secretly asked him: Why could not we cast him out?
28 And he said to them: This kind can go out by nothing, but by prayer and fasting.
Some time ago, I was reading an article stating that Pope John Paul II was unable to liberate an Italian teenager from a demon during an exorcism!
My question is this: according to Catholic teaching, why are some demons harder to cast out than others?