It's biology. Human females have a monthly menstrual cycle until their ovaries stop producing eggs.
In The Searcher and the Sword, Shuna has an issue where her menstrual cycle distracts the wolf pack, and has a thought about elven women not having that problem. I suspect that elves, with their long lives, don't menstruate as humans do, and need Recognition to cause the release of an egg to be fertilized. If that's the case, then they could conceivably eternally fertile unless they're constantly in a state of Recognition. And that could also be why it was necessary for Leetah to take a hand in Tyleet's conception.
As for Clearbrook's feeling a stillness within, it could simply be a health reason of her own. Some women are naturally barren; so, too, could some elves. Or it could be that she believes herself barren because her Recognized mate is gone and she can't conceive of Recognition with anyone else.