DIAGNOSIS The regulator isn't seated properly, or steam is escaping around it because the vent pipe is clogged or the regulator is damaged.
FIX First, ensure the vent pipe (the little metal tube on the lid) is completely clear. Use a toothpick or pipe cleaner to poke through it to remove any food debris. Then, place the regulator weight squarely on top of the pipe. It should sit level, not tilted.
IF THAT DIDN'T WORK Inspect the bottom of the regulator weight. If the hole is enlarged, warped from a fall, or has crusted-on residue, it won't seat snugly. Clean it thoroughly. If it's visibly damaged, you need a replacement regulator. An ill-fitting one is a safety hazard.
HEADS UP When the regulator is rocking gently to release pressure, it's normal for a small, controlled amount of steam to come out from underneath it. A violent, erratic "shooting" means the pressure release is obstructed—turn off the heat immediately and let it cool before investigating.