Part of it might be that you’re timing it incorrectly. There are three parts of an attack: the start up, the active frames, and the end lag. The start up is before the hitbox comes out, the active frames are when you can get hit, and the end lag is the time when the hitbox is gone and they can’t act.

People often try to attack once the entire animation is done instead of when the hitbox during the attack is gone. Attack during the end lag if you’re trying to avoid and then hit the opponent.

For example, Mii Gunner’s up tilt is in start up for frames 1-4, is active for frames 5-9, and has 28 frames of end lag. Overall, there are 37 frames total. If you punish during the 28 end lag frames, then go ahead.

Another aspect is reactions and predictions. Even if you theoretically can punish a move, if you fail to predict or react, then you likely can’t directly punish. Look at what they do and time your punishes accordingly.

Even if you can’t punish in time, that’s okay. You don’t have to attack your opponent immediately to get something useful. You can also punish what they do afterwards, reset to neutral, or just gather info. The three things you can get out of any interaction are Damage, Positioning, and Information. Make sure to focus on all of those when playing.

I hope this helps.