Guess I missed a minor edit before posting my response...
Any chance the screw holding the tab to the knob isn't tight?
Just thinkin' out loud now...
Could pull one of mine out for a comparison looksee later if no one else chimes in first.
Good thought. I just checked it and it's tight. No slippage there. I just measured the difference between the barb on the lever and the slot in the shutter and there is about .030" play which is not what I felt. I think only the the lever tip may be engaging. Maybe the shutter is supposed to have a step in it? I wonder if it's off a Micro or something.
Certainly no rush but if you do get a chance, have a look and see if the shutter on yours looks like that one. Something isn't right and it may be me.