If someone makes a reply to one of my notes I will normally zap or like their reply. It's fun to also discuss or debate. I do skip some replys because I don't know what to say.

Engagement is important.

I noticed some npubs where others will reply week after week and they never get any response or any reply to their comments.

If that happens to me I would stop following them. Its not necessary to get a response all the time but at least some acknowledgment that the person is interacting with their followers, or else it might just be a fake profile, bot, fed... etc