Oh, so it's a physics game. Still, it's not recommended to move Physics objects with non-physics behaviors and actions, they will be conflicting with each other.
There will always be a small gap between touching physics objects, I believe this is how the box2d engine works. You can try Chipmunk Physics instead:
scirra.com/forum/behavior-chipmunk-physics_t110815
Or you can try to use two objects for blocks - one invisible sprite with Physics behavior, and another visible sprite pinned to it. The second sprite should be slightly bigger to cover the gaps.