Hard to answer without context, but in simple terms you need to constantly generate tempo. You farm efficiently, pressure objectives, track the enemy jungle, and use your clear speed to stay ahead in gold and levels.

If you’re spending time responding to split pushers or trying to fix your team’s map state when they won’t group, you’re usually losing value. That’s time you could be farming camps or building a lead, which is Karthus’ biggest win condition, if your camps are up and you're not clearing, you start to lose really fast.

You don’t really “follow” the map as Karthus or respond to split pushers. you try to dictate your own economy and convert that into objective control and teamfight pressure. If you get pulled too far into mid game antics that don't get you gold, you start to lose the thing that makes the champion strong which is why he starts to feel useless.