DIAGNOSIS You're likely not aligning the lid handle with the body handle correctly. The lid must be placed so the handles align before turning.
FIX The fix is proper alignment. Place the lid on the pot, ensuring the small arrow or mark on the lid rim aligns perfectly with the corresponding mark on the pot rim (usually near the handle). Only then turn the lid handle clockwise to lock it under the pot handle. You should feel and hear a distinct 'click' when it's locked.
IF THAT DIDN'T WORK Check the rubber sealing ring (gasket). If it's missing, warped, or has food debris stuck to it, it can prevent the lid from seating and locking properly. Remove it, clean the groove, and re-seat it evenly.
HEADS UP Never force the lid to lock. If it won't lock with proper alignment and a clean gasket, the locking mechanism itself may be damaged. Do not use it, as it won't pressurize safely.