If an email bounces, the guest's row reads couldn't reach, and a Couldn't reach chip at the top of the list shows everyone it happened to.
Open their row and fix the address: Fix number, or Add mobile if they have no mobile, opens their mobile and email to correct. Save, and the invite goes again to the details you typed. Their answer, if they gave one, stays.
A bounced address is flagged in People as Email bouncing until you change it. A new address clears the flag.
If a guest says they never got it but nothing bounced, press Resend, or Copy link and send it to them yourself. Resend doesn't count as a nudge. Actions, then Resend to 9 pending, sends the invite again to everyone still pending.
Their old link keeps working after a resend, so nothing they already have breaks.