DIAGNOSIS You've got a pressure leak, most likely at the lid seal. That hissing or slight jiggle means steam is escaping faster than it can build up pressure inside. The gauge reads zero because it can't trap enough steam.
FIX First, let it cool completely. Then, 1) Remove the lid and take out the rubber overpressure plug and the metal safety valve stem. 2) Lift out the large, flat rubber gasket that sits in the lid's channel. Wash it with warm soapy water, rinse, and dry it thoroughly. 3) Before you put it back, lightly coat the entire gasket—both sides—with a tiny bit of vegetable oil or glycerin. This helps it seal. 4) Reinstall everything, making sure the gasket is seated evenly all the way around its channel with no twists or folds.
IF THAT DIDN'T WORK Check that the lid is locked down correctly. The arrow on the lid handle must align perfectly with the arrow on the pot's handle, and the lid lugs should be fully under the pot's rim. If it's even slightly off-center, it won't seal.
HEADS UP Never try to force the lid closed or open while there's any pressure inside. If it's not sealing after cleaning and oiling the gasket, the gasket itself might be old and hardened—they do wear out and need replacement every few seasons.