DIAGNOSIS This is almost always a failed or misaligned silicone sealing ring. Steam escaping prevents pressure from building.
FIX Remove the sealing ring from the underside of the lid. Wash it in warm soapy water, rinse, and dry it completely. Before re-installing, stretch the ring slightly and work it back into its groove, ensuring it's seated evenly all the way around. No twists or gaps.
IF THAT DIDN'T WORK Check the float valve (the silver pin in the lid). Make sure it moves up and down freely. If it's stuck down, clean any debris from its housing with a toothpick.
HEADS UP The sealing ring absorbs odors. For strong smells, soak it in a vinegar-water solution. Replace the ring every 12-18 months of regular use.