I just bought the Watch8 after having used a Classic4 for several years. I watched a YouTube video that showed some of the features and I liked the knock knock gesture but found it inconsistent ... until I didn't. 
For me, at least, to have the knock knock gesture be consistent, I had to do one of two things. If I'm seated in a chair, a chair with arms, I simply rest my forearm of the chair and the gesture works every time. The trick, it would seem, is to keep the forearm a bit rigid and just let the fisted hand move.
The other method which works for me, I simply use my other hand to hold the forearm in question, thus keeping the working forearm rigid. I trust this may help if you're finding this gesture inconsistent.
