DIAGNOSIS This is almost always a gasket issue. Either the silicone sealing ring isn't seated correctly in its groove, it's brand new and needs to be softened with a first ‘seasoning’ run, or you're not using enough liquid to generate the initial steam.
FIX First, make sure you have at least the minimum amount of liquid in the pot—for a model this size, that's usually around 2 quarts. Take the lid off and check the sealing ring. Run your finger all the way around the channel it sits in. It should be seated evenly with no pinches or folds. A brand-new ring can be stiff; try running the cooker with just water for 10 minutes on high pressure to soften it up before you cook food.
IF THAT DIDN'T WORK Check the float valve itself. Make sure the little silicone cap is on it and that the valve stem moves up and down freely by pushing on it with your finger. Sometimes manufacturing residue can make it sticky. A drop of food-grade mineral oil on the stem can help.
HEADS UP Never force the lid closed if the float valve is up. That means there's still pressure inside. Always wait for it to drop completely and for all pressure to be released before you try to open or re-seal it.