DIAGNOSIS This is a classic sign of food particles or starchy residue getting jammed in the locking mechanism or under the lid's rim. As it cools, it acts like glue.
FIX Don't force it. 1) Check that the pressure release valve is in the ‘Vent’ position. 2) Try running warm (not boiling) water over the lid, especially around the locking pins and the rim. This can soften any starchy gunk. 3) While holding the pot steady, firmly grip the lid handle and try to turn it counter-clockwise with a bit more oomph than usual. The combination of warmth and a firm turn should pop it loose.
IF THAT DIDN'T WORK Look closely at the locking pins on the lid. You might see visible debris. Use a wooden toothpick or a soft brush to gently clean out any bits of food from the pin slots and the corresponding notches on the pot handle.
HEADS UP To prevent this, always make sure the lid rim and the pot's top rim are clean and free of food bits before you close it. A quick wipe with a damp cloth before locking saves a lot of hassle later.