In [LOC: 29, Fire and Spirit, 418] Nynaeve makes OP history by Healing Logain of being gentled. A few minutes later, she Heals Siuan and Leane of stilling as well. What was interesting is that Logain had apparently been returned to full strength, while Siuan and Leane ended up much weaker in the Power than they had been before their stilling: [Leane speaking] "'I went by Logain's house. Six sisters are maintaining his shield, the same as when he was captured. He tried to break free when he found out we knew he had been Healed, and they said if only five had been holding the shield, he might have. So he's as strong as he ever was, or close enough to make no difference. I'm not. Neither is Siuan'" [LOC: 30, To Heal Again, 429]. Why?
First, we should note that "stilling" and "gentling" are just gender-differentiated names for the same process; in the AOL it was called "severing", and the term was applied to both sexes [Guide: 2, The One Power and the True Source, 21]; this was also mentioned by Moggy [LOC: Prologue, The First Message, 17]. There is no evidence that there is any difference in the method of cutting either gender off from the OP.
That said, there are two main theories for why the disparity with Logain and Siuan/Leane occurred:
Jordan confirmed that it is the second case in a blog entry, "...the Healing of stilling must be done by the other gender to be fully effective. A woman Healing a woman or a man Healing a man results in less than full restoration. It all ties into that theme I keep harping on. Men and women have to work together to be their most effective. And while the weave used by Flinn for Healing is not exactly that used by Nynaeve, either would use the same weave on a man or a woman."