‘And he says to them, “Because of your little faith. For truly I say to you, If you have faith as a grain of mustard seed, you will say to this mountain, ‘Remove hence to yonder place', and it will remove, and nothing will be impossible to you.” '
Jesus explains that the reason that they had failed was because of the insufficiency of their faith. That it was quality of faith and not the size of it that mattered comes out in the comment that followed. If faith is of the right quality then only the tiniest amount is required, faith the size of a mustard seed, and the mustard seed was, in Palestine, the smallest of all seeds used by Galilean farmers and proverbially small. But with the right quality of faith even mountains can be removed by a word. Indeed, Jesus stresses, with the right quality of faith nothing is impossible. So what needs to be developed is faith, and this can only be developed by regular prayer. The need to build up faith is Matthew's emphasis.
It is in Mark 9:29 that He makes clear that such faith is developed by much prayer. We are never told how much the disciples prayed, but from this it was clearly not enough. Jesus was not, of course, advocating actually removing mountains. That would hardly be within God's will, and believing prayer must be within His will (1 John 5:14). He was speaking about every kind of difficulty. Compare especially Zechariah 4:7. ‘Removing mountains' was a proverbial figure of speech for overcoming great difficulties (compare Matthew 21:21-22; Isaiah 40:4; Isaiah 49:11; Isaiah 54:10; Mark 11:23; Luke 17:6; 1 Corinthians 13:2).
‘And nothing will be impossible to you.' Nothing would be impossible for the one who truly believed God. This was because of the greatness of their God (see Matthew 19:26). His point is that nothing is too hard for the Lord (see Genesis 18:14; Job 42:2; Jeremiah 32:17; Jeremiah 32:27), and therefore nothing is impossible for the one whose faith is true.