It's likely that that guest hasn't finish their RSVP.
With our Chip-In feature, guests begin their RSVP in Partiful, and then are navigated to a payment platform to transfer the payment. Once they've done that, they need to navigate back to Partiful to finish RSVPing.
In this case, you should reach out to this guest to let them know to finish their RSVP. If you don't have their contact info, you can DM them in the Partiful app!