DIAGNOSIS Gunk—starch, grease, or tiny food particles—has gotten inside the valve mechanism and is gumming up the works. It's a common issue after cooking starchy foods like pasta or oatmeal.
FIX 1) Make sure the pot is cool, unplugged, and depressurized. 2) Look at the black steam release handle on the lid. You can remove it for cleaning. Pull it straight up and off. 3) Underneath, you'll see a silver metal valve stem. Wipe it clean with a damp cloth. 4) Now, look inside the black plastic handle. Rinse it out with hot water. A small brush (like a clean toothbrush) can help dislodge any debris inside. 5) Dry everything completely and snap the handle back onto the stem. It should move more freely.
IF THAT DIDN'T WORK Check the silicone cap that sits under the steam release assembly on the inside of the lid. Remove it (it just pulls off), wash it, and make sure it's not deformed. A misshapen cap can put pressure on the valve stem.
HEADS UP Always move the steam release handle firmly but gently. Forcing it can break the plastic. After cooking starchy stuff, do a 'steam clean' by putting a cup of water in the pot and running a 5-minute steam cycle to help flush the valve.