DIAGNOSIS The float valve (the little metal pin in the lid) is either stuck down by food gunk or there's a major leak preventing any pressure from building in the first place.
FIX After letting the cooker cool completely, gently poke the float valve with a wooden toothpick or the end of a paperclip. Don't use metal that could scratch it. You're trying to dislodge any starchy residue or pea that's keeping it seated. Then, push it up and down a few times under running water to flush it out. Dry it, and before your next cook, do a water test with just a couple cups of water to see if it now rises.
IF THAT DIDN'T WORK If the valve moves freely but still doesn't rise during a test, you have a bigger sealing issue. Follow the steps for the constant hissing problem—the core leak is likely from the main gasket or lid lock.
HEADS UP The float valve is a secondary seal and pressure indicator. If it's not rising, do not assume the cooker is at pressure. Rely on the pressure regulator weight rocking gently as your true sign it's ready. A stuck-down valve can also be a safety hazard if it's supposed to drop as a pressure release.