If it's not the amount of channeling she's doing, and it's not the Oath Rod, is it the kind of channeling? Maybe it takes the stress and adrenaline of fighting with the Power to push you to your limits? Or it's something to do with the lack of experimentation and innovation in their weaves? Are you better off working out your own weaves than learning someone else's? Or maybe it's to do with the testing process? The Arches or that amnesia weave they hit you with for the Aes Sedai test?
Edit: oh, or, is it men? Do you need to link and channel with saidin as well if you want to reach your limits?