DIAGNOSIS Yeah, this one's common. You've probably got food debris or mineral deposits jammed in the alignment grooves or under the lid's locking lugs.
FIX First, don't force it. Make sure the cooker is cold and depressurized. Take the lid off completely and inspect the inner rim of the pot and the underside of the lid. You're looking for little bits of rice, bean skins, or hard water crust. Clean both mating surfaces with a soft brush and warm soapy water. Pay special attention to the small notches (the alignment grooves) on the pot's rim. Rinse thoroughly, dry, and try locking it on without the gasket first to see if it rotates smoothly.
IF THAT DIDN'T WORK The silicone sealing ring (gasket) might be mis-seated or swollen. Remove it completely from the lid, wash it, and let it dry fully. When it's cool and dry, stretch it slightly and re-seat it evenly in its groove all the way around. A swollen, old gasket can prevent proper locking.
HEADS UP Never try to use the cooker if the lid isn't locking freely and fully. The pressure won't build correctly, and it's a major safety issue. If the locking lugs on the pot itself look bent or damaged, stop using it.