The steam thermostat controls steaming temperature and consequent pressure. They may "drift" and change state (turn off) at a lower temperature, but that would be my second guess. My first guess is that the steam nozzle is clogged. You can use a safety pin to clean it out. The steam valve controls steam flow; it has a nylon seat and an o-ring. If it's not the nozzle or thermostat, I'd suspect that the valve seat is somehow jacked.
That's a (exploded) parts diagram, not a wiring diagram, not a schematic.
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