Cindy Moon (Silk) have Eidetic memory, so she is able to recall and learn Spider- Man (Peter's) moves easier and she was already an athlete (hokey player) before she was bitten. That is why studying videos of Peter's fight worked better for her than it did for Miles. Miles learned early on that he couldn't mimic Peter's moves and chose to follow his own instincts when he fought.
Miles was a normal 13 year old kid, when he got his powers, he was smart but not a genius like Peter.
Most of the time Mile's venom blast works, because he catches his opponent off guard and they become distorted, as was the case here, and it was not a complete knock out. As was stated before there where times when he used it and it didn't work.