The one innovation by an old-school sister was the detection of male channeling, and that was obviously and blatantly inspired by the rise of a competing group! Even when male channelers were the greatest menace to the world, no one came up with it, once the Breaking was over and they became an individual nuisance. Cadsuane's & Nynaeve's ter'angreal sets are said to date from the Breaking and imply that such techniques or equivalents might have been known at that time, but once male channelers as a group were no longer a threat to the collectivized Aes Sedai, the technique was lost and never regained, presumably due ot the low priority. Human nature being what it was, it took the assassination of two sisters by a weilder or saidin to drive the recovery of that ability. Nothing concentrates the mind like the prospect of being hanged the next morning, as the saying goes, and it appears that the possibility of being the next victim of an undetectable attack has a similarly salubrious effect. Now, there is nothing to say the sister who developed it had not been working the problem all her life, but the fact is, she only mastered it after a couple of deaths made it abundantly clear that an immediate, clear and present threat existed.