First thing you should do, if you haven't already, is upload the latest firmware to your printer from here
download/file.php?id=3543
If that doesn't work, try the following steps, one at a time:
1. Make sure that the Z axis drives are at the same height. Measure between the Z stepper motor bracket and the bottom of the Z axis nut. If distances aren't equal the drive will bind.
2. Lightly lubricate the Z axis drive rods
3. Check the power supply lead is pushed home on the control board.
4. Check that both of the Z drive leads are plugged in properly at both ends.
5. Make sure that the stepper drivers (A4988) are plugged into the control board correctly and that the jumpers (underneath the drivers on the control board) are in the correct position.
6. Swap the Z stepper drivers (A4988) over, does the fault change/move? If it does, then one or both driver boards are either duff or need setting up.
7. Swap the drive connectors for the 2 stepper motors at the control board, does the fault change/move? Possible duff lead or connector
Post back with the results