DIAGNOSIS If the lid spins freely without catching and locking, the alignment is off. This is often because the inner pot isn't sitting correctly, or the silicone ring is out of its track and physically blocking the lid's rotation.
FIX First, remove the lid entirely. Take out the inner pot and then reseat it firmly in the base, making sure it's sitting flat on the bottom. Next, inspect the large silicone sealing ring around the underside of the lid. Make sure it's fully seated in its circular groove all the way around. If it's popped out in one spot, it will act like a speed bump and stop the lid from turning.
IF THAT DIDN'T WORK Check the metal float valve on the lid. If it's stuck in the up position (maybe from last use), manually push it down until it clicks into its down position. The lid cannot lock if that valve isn't fully down.
HEADS UP Always align the arrow on the lid with the unlock symbol on the base before you try to close it. Then turn the lid to the lock symbol. Forcing it when it's misaligned can damage the plastic tabs.