DIAGNOSIS: The sealing ring isn't seated properly, is damaged, or food debris is on the sealing surfaces. A perfect seal is critical for pressure.
FIX: 1) Unplug and let the cooker depressurize and cool completely. 2) Remove the lid and take out the silicone sealing ring. Wash it thoroughly and check for any nicks, tears, or permanent folds. 3) Clean the groove the ring sits in and the rim of the inner pot—any tiny bit of food can break the seal. 4) Make sure the ring is fully seated back in its groove all the way around before closing the lid. Listen for that solid click.
IF THAT DIDN'T WORK: The sealing ring can lose its elasticity over time. If it looks worn or feels less springy, it needs replacing. Ninja sells replacement rings specifically for this model.
HEADS UP: Always ensure the steam release knob is set to 'SEAL' and not 'VENT' when trying to build pressure. It's an easy mistake to make.