DIAGNOSIS This is a jammed or failed Quik-Lok blade clamp mechanism. Debris (metal shavings, wood chips) is packed into the clamp, or the internal spring or pin is broken.
FIX First, try to clear debris. Use compressed air to blow into the clamp from all angles while working the release lever. If that fails, apply a penetrating lubricant (like WD-40) into the clamp, let it sit, then try the lever again.
IF THAT DIDN'T WORK The internal roll pin that actuates the clamp jaws may be sheared or the spring may be broken. This requires disassembly of the shoe and clamp assembly to replace parts 02-06-0138 (spring) and 02-06-0136 (pin).
HEADS UP Do not use excessive force on the release lever. You can break the plastic housing. If debris is the cause, clean the clamp regularly after use in dirty conditions.