DIAGNOSIS The primary pressure regulator (the small weight that sits on the vent pipe) is likely missing, or the sealing ring is damaged or dirty.
FIX First, locate the pressure regulator (the small, often black, weighted piece). It must be placed securely on top of the vent pipe coming out of the lid. Without it, steam escapes too fast to build pressure. Ensure it's clean and the vent pipe hole is clear.
IF THAT DIDN'T WORK Inspect the main rubber sealing ring. If it's cracked, hardened, or has bits of food preventing a seal, it will leak. Remove it, wash it thoroughly, and ensure it's seated snugly and evenly back in its groove around the entire lid.
HEADS UP Always start cooking on high heat until the regulator begins to rock/jiggle, THEN reduce heat to medium-low to maintain a gentle rocking. High heat throughout will boil off liquid too fast and can cause burning.