Think about all of Jesus' miracles. All of the stuff that the disciples have seen. Mind-blowing stuff. Then 4 days before he is cruficied, they see Jesus curse a fig tree and it dries up immediately, and they can't figure out how he did it.
It's like they constantly forget that Jesus is God and can do anything that he wants. They've seen it constantly, so why would they be so amazed by this.
This is so similar to my faith, though. I mean, I constantly am amazed by what God has done for, through, and with me, and yet I never think he can do it again. I always feel like I'm struggling to stay afloat and yet God is constantly showing me that he is perfectly capable of teaching me to swim.
One of my favorite characteristics of the disciples is that they were so dumb. They never understood what Jesus was saying or at least the true implications of it.