the heater valves are mechanical.
Often, "sludge" from dirty / old coolant clogs and they either can not open or stick closed or open, etc.
A lot of times turning them on and off enough times can temporarily (if your lucky, permanently) dislodge the sludge and restore normal operation.
Aside from that, sharp bumps, jolts, etc can help it and sometimes it's just pure luck.
It's very possible that one or the other will jam again, but if not - glad to hear that your luck worked out.
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