Loose earth?
Got caught out on that one for almost an hour one day before I figured it out.Is there a switch for ark or tig?
Happened to me. Took a long while to figure it.
Mate of mine is a welder and took his back to the shop only to have it pointed out!!
I would check the leads as has been mentioned earlier, all might look fine on the outside!!There is a switch for arc and rig nd it’s turned to arc