It's commented on not long before - Cadsuane talking to that man in Bandar Eban and not hearing about any positive effects to balance the negative ones - and so I thought that was the reason why it failed to work on Tuon. Although I guess that doesn't entirely work, since his ta'veren first *does* work, but then somehow doesn't - it really looks like Tuon is affected by the ta'veren twice. Once into almost accepting it, but then again into refusing when she was about to accept.