A lead stays unmatched when LeadSight can't find any Xero contact that resembles it closely enough to suggest — for example, if the name and email are both completely different from anything in Xero, or if that lead simply hasn't converted to a paying customer yet.
If there's a partial resemblance — similar name or email — it will usually show up in your Matches queue for manual review, rather than being silently dropped.
If you believe a lead should have matched and it hasn't appeared anywhere, check that the corresponding invoice has actually been created and paid in Xero.