1) It might be disabled under your account. Go to your contest settings and make sure it's enabled.
2) Their email might have been faulty. We verify emails exist before sending them.
3) They might be referring themselves to try and cheat the contest. We don't send confirmation emails to these users.
* We don't send confirmation emails to users who have signed up with the same ip address multiple times.
4) If you're testing confirmation emails yourself, only the first confirmation email will get sent. This is to prevent bad IPs from requesting emails constantly and lowering our deliverability score. To remedy this, you need to delete the entry you just created.
5) If you setup SMTP settings, make sure your SMTP settings are correct.