There are a few reasons why this might be happening.
#1 - If you're testing your contest, note that every email will only come once to the same IP address / email address (meaning confirmation email will only come once, even if you test it multiple times). This is to prevent spammers from destroying our email reputation.
To override this, you need to delete your "test entries" in the "winners / entrants" tab.
#2 - If you've installed SMTP, please ensure your SMTP settings are correct and that you are on the right plan at your provider. If for example, you are over your limit at your provider, they may not send out your email.
#3 - Users marked as cheaters will not receive any emails.
#4 - Users who've signed up multiple times to the same contest (based on IP & email address) will not receive any emails.
#5 - The email might be in the spam inbox. We suggest adding your own SMTP details in order to maintain the highest deliverability.
#6 - If you set up your contest with "confirmation emails" the end user must confirm their email before they receive any further emails (like reward emails).
#7 - Reward emails do not get sent if you are manually adding / subtracting entries (points) to a user.