But you're on the wrong track. There's no reason to think this person didn't channel much. There was another reason why she didn't reach her potential.
I also find this kind of thing more interesting than strength rankings, which is why I dropped the hint. I think it can be figured out; it's not difficult really; it just requires out-of-the-box thinking.
Was it Siuan? She had a lot of politics to deal with which could have cut into her practice time, and she was Stilled before she was old enough that we could take age as presumptive evidence that she was at full strength.
She's been ruled out already ... probably doesn't fit the "old school and old" criteria.