When Jesus entered the house, the disciples asked Him privately, “Why could we not cast it out?”
The disciples humbly ask about the reason for their failure. This is a good sign, because they do not justify themselves and do not blame others, but want to understand the cause of their failure. Their question is personal and private, which shows their humility and readiness to learn. Their amazement is also understandable, because earlier they had received authority over evil spirits (Mark 6:7, 6:13), but now they had faced their powerlessness. In spiritual ministry, defeat can become a great teacher if a person turns it into a humble question before Christ. They had felt their limitation and understood that outward calling alone or past experience is not enough.