The locking wire is too tight.

Loosen the locking wire tension enough to provide a full engagement of the lock bar in the locking slots of the adjuster lower channel.

The adjuster lock bar spring is disconnected or broken.

Connect the spring or install a new spring.

The adjuster lock sticks or binds.

Lubricate the lock bar pivot.

Eliminate the cause of the binding or replace the adjuster if the bar is still binding.