Sated Devour Vi + E

Not sure if it's only with Sated Devour and not regular Devour, but occasionally Vi's E will prep properly when used, but then it does nothing on hit, usually when I E immediately after an auto attack. I also have BotRK and AS runes so my attack speed is quite high, not sure if it happens at lower attack speeds. This happens 1-3 times every game.