collision detection – With Unity, why this character with a rigid body and child element colliders doesn’t collide with the floor and go through it?

I got an issue with my character going through the ground and actually going through all other elements with a collider. I’m going to try and show as much as I can of the current state of things.

My character (Human) has a rigidbody component with gravity on. He doesn’t have directly any collider on it (only on child elements).

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As you can see, an armature is defined on the Human, each part of the armature has it’s own rigidbody as well as a collider, as shown in the below picture :

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None of those colliders has IsTrigger checked. The Human has a whole doesn’t have a collider as this would overlapse with the colliders defined on each part of the Armature.

Now I’m trying to put my characters on a floor object with a box collider. The box collider of the floor is roughly half the height of the character, so it can’t be an issue with discrete collision detection. My character at t=0 has no speed and only the acceleration of gravity.
Here’s how the floor looks :

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Both my character and the floor are in the Default Layer and the box is checked in the Layer collision matrix.

Is there anything you think of that I forgot ?