Sounds to me as if the lever mechanism has failed, and the
lever release is just not responding to the trigger inputs into
the mechanism.
Try dropping the panel to see if the little release pawl shows
any sign of movement when the trigger is pressed.
Might need a new,( Overhauled ) assembly.
ie. It's just plain Knackered! And broken internally.
Had a similar problem years ago. But that was a DC-8.