Can you give a bit more info about your setup. Eg software, plasma machine, software and wiring.
You could have a wiring fault that means you are not getting an ArcOK signal
Your sensing circuit may have a minimum current draw that is less than the mimimum spec for the relay in the machine
or it could be that your machine is not establishing a valid cut and shutting down.
You have not given enough info for us to help.