With the tension released you should be able to flex the clamp with your hand. If you can't either the pedal is broken or you turned the tension screw the wrong direction (unlikely since you had the 105 version).

It could also be a damaged or incorrectly installed left cleat.

I routinely clip shoes into Look pedals by hand on my wife's bike with no serious effort, and she has the tension set in about the middle. If you can't do the same it should at least show you what is not working and you can go from there.

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"...the street finds its own uses for things"