What control software are you using.? Like Micheal says it very much sounds like the machines setup wrong.

It's not uncommon for the Z axis to be set to a different number of steps to the X & Y axis in the control software.?
Usually this is because the Z axis screw use's a smaller pitch so requires more steps to cover the same distance. . . . So if for instance your X & Y use 10mm pitch screws and the Z Axis use's 5mm then it will move exactly half the distance of the X & Y for the same number of steps so possibley some ones used the same settings for all Axis.?

Easy 2min fix but first if you can give a few more details on the drives and how they are setup regards micro stepping then it may be better to change the micro stepping at the drive or just change steps setting in the control software.?
If your not sure how they are setup then some good pics of the drives will help along with make and model. Also the control software used IE Mach3, EMC2 etc