This was not the kind of unbelief the disciples had when they tried to cast the demon out of the young boy. They were not hardening their hearts against God's plan. No, they faced the second type of unbelief passive unbelief. The disciples were truly believing God, but their faith was nullified. As soon as they could, they took Jesus aside and asked in all honesty why they could not cast out the devil this time as they had before.
Passive unbelief comes from over-occupation with legitimate things of life.
Necessities are not bad, wealth is not wrong, and things are a legitimate part of life. But if we spend too much time concerned with them, our fruitfulness is choked out. God wants our time to be taken up with prayer and the Word of God. He knows that His Word is our source of life. Passive unbelief takes us away from the Word as we fill our life to overflowing with other activities.