How compacted was the soil when you dug the hole to plant them? Did you have to use a pick? Are you sitting on top of clay with just a few inches of topsoil? If that's the case, the roots are having a hard time getting established, and the growth on top is all they can scrounge up nutrients for.
Fortunately, there are things you can still do to improve the soil for them. One is
hoserkultur, an after-the-planting method of putting in a hugelbed. You could also dig a trench near to these trees and turn it into a hugelbed. When the roots of the trees do break through to some buried organic matter, the growth of the trees will take off and you will no longer be disappointed by them.