"Maybe they don't supply enough O2 flow if the cabin pressure is above 30,000 feet?"
Open a bottle of seltzer? you get bubbles. I recall from my AF Aircrew training at 30,000 ft one needs pressurized oxygen forced in the lungs otherwise the oxygen "boils" out of the blood. Passenger oxygen is not under such positive pressure. In fact the oxygen masks are designed to dilute the oxygen (plastic bag).
Maybe at 300,000 feet.