There are a few people who are amnesiac. Those who have regular, periodic amnesia will constantly have fresh chances to attempt to will magic, and each time they either will or will not succeed, meaning on some days they will be able to will, and on others they can’t. Then there are those who are amnesiac because they suffered some brain injury that wiped out their memory. If it’s someone who had previously been impotent, this gives them a rare chance to become willful. If it’s someone who had been a willful, well now they’re untested and they might fail – regardless of whatever people might tell them as to whether they had succeeded before or not, since their subconscious can’t draw on any such memory.