I've opened the top, fully released the switch mounts from the carriage, set the coupling nuts properly, reassembled the mounts and confirmed switches both function. BUT- the z-axis will move in a positive direction, but when I try to move negative, the machine makes a slight 'clunk' and fails to move in the slightest. I've had the machine a few months. Enough to start to feel comfortable, but this has me stumped. I've Google'd the 'net until their support group called and asked me to stop, I was overheating their servers. Seriously- any suggestions? Thx.