DIAGNOSIS Food particles or starchy foam has gotten into the valve mechanism and hardened, jamming it. This happens especially with beans, rice, or thick sauces.
FIX Let the cooker cool completely. Unscrew the steam release valve from the lid (it twists counterclockwise). Soak the valve in hot, soapy water. Use a pipe cleaner or a small brush to clear the tiny hole in the valve's stem. Rinse thoroughly and screw it back onto the lid firmly.
IF THAT DIDN'T WORK Check the lid's opening where the valve screws in. Use a toothpick to clear any debris from those threads. Also, ensure the little silicone gasket inside the valve isn't missing or deformed.
HEADS UP Always do a quick release immediately after cooking for starchy foods to minimize this. Never force the valve if it's stuck while the pot is under pressure—let it cool naturally first.