To me, I feel like it happens alot more as the rounds in your battl progress. For example, hinata's(3 star) attack normally has a higher chance of knockdown at round 2 than in round one. Or maybe it's just luck? To b honest, I feel like it actually is more based on chance, cuz i also have ao, and his attack doesn't cause low float up until around round three, and other times round 1 and 2.