Actually, writing that out made me think. These are just some random ideas here….
When Sayaka is talking to Kyoko in the train station shortly before transforming into Oktavia she states that once a girl has suffered enough despair to balance out the hope that she wished for that it's all over and witchdom then ensues. That would imply that the transformation to a witch actually requires the experience of suffering, rather than simply having one's soul gem go black from using too much magic. If that is true then a girl who was in a coma, lobotomized, etc, wouldn't actually become a witch because she wouldn't actually be suffering. A similar situation might happen if the girl in question was drunk, high on drugs, or otherwise was unable to feel any suffering. Or if the magical girl was some kind of masochist or for some other reason didn't "suffer" the same way that normal people do?
Then again, it was also stated by Homura that once the soul gems "turn black" then the girls are reborn as witches. So perhaps a girl could become a witch for either of two reasons:
1) suffering, as Sayaka stated
2) using too much magic