DIAGNOSIS The float valve (the little metal pin in the lid) is either stuck down by food debris or has a faulty spring mechanism.
FIX Let the cooker depressurate completely and cool down. Push the float valve up and down with your finger a few times. You should feel it spring back up. If it's sticky, there might be starchy residue (rice, beans) around it. Use a toothpick or a soft brush to gently clean around the base of the valve and the hole it sits in, then rinse the lid.
IF THAT DIDN'T WORK Inspect the silicone seal around the float valve. Sometimes it can become misaligned or swollen, pinching the valve and preventing it from rising. Reseat the main sealing ring properly, as that can affect the valve's operation.
HEADS UP Never try to force the float valve up while the cooker is pressurized. If it's not rising, it's a safety indicator that something is wrong. Do not continue cooking if the valve remains down despite obvious steam release—it means the pressure sensing is faulty.