I had a similar problem and battled it for days (with Prusa I3 X).
It seems a very common problem (The Z motor is more complex than the others, it has two motors. The left and right sides of the Z axis have to be at exactly the same height. It has to carry a heavy load against gravity)
http://www.geeetech.com/forum/viewtopic ... 22&t=16218
http://www.geeetech.com/forum/viewtopic ... 17&t=16890
http://www.geeetech.com/forum/viewtopic ... 17&t=16765
Here is how it was solved in my case:
In my case the two Z motors were making a grinding noise, when trying to change the Z-axis location through the Repetier host program. The Z axis did not move. This was the behavior immediately after the assembly has been completed.
Remember to power off the motors when you turn the threaded rods by hand (turning the Z couplers at the bottom of Z axis is easier than turning the threaded rods themselves). When the motors are on, they resist moving the threaded rods by hand. (The motors are also affecting each other when they rotate).
Remove the top acrylic plates for Z axis and make sure the smooth rods are moving easily when you lift them and then lowering them back to their location.
Make sure with a caliber that both sides of Z axis are at the same height. This can be done by measuring the distance between the two acrylic plates as demonstrated in the photo below. If they are not level, there will be more resistance to their movement. Rotate the coupler in one side by hand to lift or lower that side.
Change the Z-coupler location, probably lift it a bit (I have seen in photos that they were a bit above the acrylic plate that lies beneath them). When the couplers are lifted, the bottom side of the coupler is connected firmly to the motor shaft, and the top of the coupler is holding the threaded rod.
Make sure the voltages between the screw and GND of the step motors drivers is in the range 0.9-1 V (mine was about 0.8v). It increases the power to the motors, so they can overcome higher resistance. (Search the forum for explanation how to do that).
Is it not moving only when Z axis is at 0 location? Try rotating the Z couplers by hand to a different Z location, and then test whether the motors can move at that location from the Repetier Host program.
Changing the couplers location, had fixed the initial problem of stuck Z axis immediately after assembly. There were still other problems with Z axis movement.
In my case after the initial fix, the Z axis could only move up to location 40 mm, and at that location, again the Z axis motors made a grinding noise and stopped rotating.
Looking carefully at one threaded rod I have noticed that at that location, there is an imperfection at the thread of the threaded rod. This imperfection caused a slightly higher resistance when that imperfection was inside the Z axis nut.
This was solved by applying lubricant (WD 40) to the rods at the imperfection location and above and below it, and then rotating the couplers by hand back and forth several times in order to move the imperfection several times through the Z axis nut. After moving the imperfection through the Z axis nut by hand, I started to move the imperfection through the Z axis nut from the software. After about 20 such movements, the Z axis begun to reliably move through the imperfect point.
There were several such imperfect points at different heights of Z axis. Using the same method of lubricating and going through these weak spots back and forth several times solved the inability to move through these locations, and eventually the Z axis was reliably moving up and down the full 15 cm!