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The height is simply too high. You can not physically go any higher without shortening the valve knocker, but if you do that, it's the exact same thing as leaving the length the same and keeping the small shim on.
It's not a mainspring strength issue. If the valve strike face is too high, it will push the valve knocker *up* when you insert the magazine, and when you rack the slide / cock the hammer, a high strike face will not give the valve knocker enough clearance to drop down in to firing position.
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