Almost always one of four things.
1. Check the address you typed
Open your event and read the guest list back. One wrong character is the most common cause, and it looks exactly like a delivery failure.
2. Ask them to check spam
An invitation from an address they've never seen is what spam filters are built to distrust. Ask them to look in Spam, and Promotions on Gmail. Marking it not-spam fixes future emails too.
3. Check whether it bounced
If their mail server refused the message, we record that and stop trying. Your event says so on that guest's row, with a box to correct their address and resend.
Once an address has bounced, we stop sending to it — on this event and later ones. You can fix their email address or send them the event link yourself.
4. Give it a few minutes
Mail is usually near-instant but not guaranteed. A slow receiving server can hold a message for a while.
Still nothing?
Copy your event link and send it to them yourself, in an email, text or chat.