Misha, it might help if I confirm, when you separate the body from the plunge head and depress the black spring loaded retainer, the green handled adjuster moves very freely, with no resistance at all. It will 'flop around' in the body.
If yours doesn't do that, I'd suspect damage from an impact or someone really forcing the lever without knowing what they were doing. Separate the plunge head from the body and check yours to see if it moves completely freely. If it doesn't, there's a reason for that and looking at the good condition of that tool, I don't think it's corrosion for example. What you describe sounds like the shaft on the part you're wetting with WD40 is bent and looking at mine, I could see how that could happen in the wrong hands.