If your vehicle is showing seatbelt locks up constantly during normal driving or leaning forward, here are the most likely causes and the parts you'll need to fix each one.
Over time, dust, lint, pet hair, or spilled liquids can accumulate inside the seatbelt retractor housing. This causes the sensitive gravity pendulum or centrifugal clutch to stick and engage the lock prematurely.
The seatbelt fabric may have folded over and twisted inside the B-pillar trim or the retractor itself. This causes the belt to bind on the guides, tricking the mechanism into thinking it is being pulled rapidly.
The internal spring, gear mechanism, or inertial pendulum inside the retractor has simply worn out or broken from years of use, making it hyper-sensitive to normal body movements.
Seatbelt retractors are highly sensitive to gravity and must be mounted perfectly vertical. If the retractor was removed during interior work or body repairs and bolted back in at a slight angle, the pendulum will be pre-tilted and lock constantly.
If the vehicle has an extreme aftermarket lift or drop on only one end, the resting angle of the entire vehicle changes. This tilts the gravity-sensitive pendulum in the factory seatbelt retractor, causing it to lock at the slightest deceleration.