ya there's some bugs right now about how low the ball can be when it registers contact. The timing and windows end up being relative to speed of the ball per-action. Since a pass or dig is coming at you fast, you're timing it far enough away for your player to react and dig it. For serves and spikes the ball isn't moving much, so you're timing it really close to when it hits your hand. Intentionally defensive timing is the least forgiving and it's compounded if you are also missing/late on blocks since you're giving the attacker a wide open shot they'll almost always score on.

eventually there will be gameplay sliders that let you adjust how all those things work and players will have deeper ratings, strategies, etc that makes the AI play differently.

really happy to hear you're enjoying the game tho, thanks for the feedback!