If you are faster with timed flutter vs a 2bk, likely the legs are splaying or scissoring on 2bk due to an imbalance. Scissor/splaying legs is the body's reaction to stabilize the vessel. Flutter or 6bk doesn't have a much time to splay and adds stability by virtue of more kicks. A visual/focus you can use to help identify the imbalance is "kick inside the bucket" in 2bk and 6bk, compact the kick. Like kicking inside a 5 gal painters bucket, don't kick outside the bucket.