DIAGNOSIS Pressure is still trapped inside. The float valve is a physical safety lock. It will not drop until internal pressure equals atmospheric pressure.
FIX Do not force the lid. Ensure the steam release handle is set to "Venting." Wait. It can take 10-15 minutes for natural pressure release. The valve will drop with a click when safe to open.
IF THAT DIDN'T WORK If the valve is mechanically jammed by food, use a wooden chopstick to gently push down on the metal pin of the float valve once the pot is cool and depressurized.
HEADS UP Forcing the lid can warp the sealing ring rim. Always confirm the float valve is fully down before attempting to turn the lid.