Can you see any deformation in the nozzle? Is the electrode arc point in the dead center?
I had a similar issue that after about 10 minutes or so cutting 3/8" mild steel I'd start to get and angled kerf on one side. The electrode showed wear off-center which i guess caused the arc to be off-center and in turn caused arc wear on one side very quickly. It turned out to be a bad swirl ring. A quick replacement and I was surprised and happy at the same time.
I wondered why Hypertherm considered the swirl ring to be a consumable. I had never replaced it on this system.