For some reason, in the end we don't get to know why the stone didn't work on the MC.
The MC touched the stone and tried to manifest some powers but nothing happened then Rakess called Olera and she checked the stone but it seemed to be working fine, then she checked the MC and the focus shifted completely to the MC's unknown innate powers and we're never told any explanation for how or why those powers made the stone not work or even if it was those powers.
I assume every challenger gets a power from the stone and not only the ones that don't have much, otherwise deciding where is the cut-off would be a bitch. Maybe, because of her powers, the stone consider the MC one of Olera's kind and it doesn't work on them for whatever reason.
I just think the stone thing wasn't resolved nearly clearly enough.