DIAGNOSIS That 'little shield' over the float valve is a critical anti-clog device. Beans, rice, and foam from cooking can get trapped in and under it, which can prevent the valve from sealing or dropping properly.
FIX It's designed to come off for cleaning. Once the pot is cool, locate the float valve in the center of the lid. The shield is usually a separate metal piece with two little ‘legs’ that slide into slots. Gently pinch those legs together or push up from the bottom, and the whole shield should lift off. Wash it, the valve, and the area under it with a bottle brush and hot soapy water. Rinse well and let it air dry before snapping it back on.
IF THAT DIDN'T WORK If the shield is truly stuck with baked-on gunk, soak the entire lid (with the shield on) in warm, soapy water for an hour. That should loosen everything up enough for you to remove and clean it properly.
HEADS UP Never put the lid with its valves and seals in the dishwasher. The intense heat and detergent will degrade the silicone seals over time, leading to leaks. Hand-wash only.