Owen,
2mm distance between metal sheet and nozzle is ok.
Try with 3 bar oxygen pressure.
Adjust the distance between nozzle and focusing spot (not nozzle and metal) at 3mm. (look the attachment)
In the picture you can see 3 cases.
Case 1...the focusing spot is under the metal. (wrong case)
Case 2...the focusing spot is above the metal. (wrong case)
Case 3...focusing spot is in the metal. (correct)