A compliment you forgot giving might be the one sentence someone replays every time they doubt themselves.

Praise is weightless to give and heavy to receive. A sentence you said in passing may be stored and replayed for years by someone who needed evidence they were doing all right, and you will have no memory of it. The asymmetry is enormous and invisible. You almost never learn which of yours landed.

Say the good thing out loud when you notice it instead of assuming they know. Be specific about what you saw, since general praise is easy to wave off. Do this with the people who seem confident too, because that is usually where nobody bothers.