This generally happens for a couple of reasons.
1 - The user provided a typo in their email address.
2 - The user has signed up multiple times with the same IP address.
We don't send duplicate emails to the same IP in order to prevent spam emails from getting sent emails. Undeliverable emails unfortunately hurt our sender reputation, which is why this is in place.
3 - The reward / opt-in bribe unlocks at zero or 1 entry.
This is to prevent a contestant from getting multiple emails sent on one signup.
In order to make sure they receive their opt-in bribe or reward email, make sure you customize the confirmation email to INCLUDE the details to redeem their reward or opt-in bribe.
With that in mind, you must also have enabled the sending of confirmation emails in your settings tab.