Why do you think few knew about it? Most sisters who've had to interact with her over the years could not fail to notice her potential was still greater than her strength. It's second nature to Aes Sedai to notice such things, since this governs social interactions and power relations between them
Theorically, many would not know, because with over 900 AS there are always many you don't know or never interact with. That's especially true of sisters like Moiraine who spent a lot of time out of the Tower and thus has never yet met many sisters.