Both X and Z rods not spinning but motor whirring?!

Hello, sorry for sounding really thick but I've just bought and built a SC300/2. However, I'm having a bit of an issue getting the X and Z axis working. The rods will turn when I take them apart but when I thread them through the lead nut screw and tighten the machine they just won't spin, therefore won't work properly. You can hear the motors trying but the rods just dont move. I've taken them apart and put back together several times to adjust but still just the motor whirring but no movement. Any ideas?
Thanks.