I seem to keep having a problem with moving objects up and down. Normally I would press "Z" and the mouse button to move things up, but for whatever reason it stops working from time to time. I've had this problem in the past and fixed it, but can't remember what I did.

Can someone please refresh my memory and help me fix this?

Thanks.

Edit: After posting this my entire CK went crazy. Nothing was working right. Not sure what happened. I just decided to uninstall and reinstall.