I played Fizz since season 2, and every time I would use Q, sheen activated and apply bonus damage. In season 4, however, this was changed and bonus only applied in subsequent hit. This doesn't make sense, since it clearly states in Q's description that and I quote:
> The ability will also apply on-hit effects.
Since you activate Q, and while activated you hit your target, it is only logical that bonus applies with Q hit, not afterwords.