It is most likely just a display issue on your client. Maybe if you view a note you know for sure you zapped and just stay on it for like a whole minute, it might show up 🤷‍♂️

This kind of thing is very common using damus

Check out nostr:npub1xtscya34g58tk0z605fvr788k263gsu6cy9x0mhnm87echrgufzsevkk5s ‘s zap contest thing (linked in his status currently) for example, and you might find that there are a lot more zaps than first appear