There are several possible causes: the e-mail address may be incorrect or blocked by anti-spam (especially if it is a company e-mail).
Go to the Monitor my Secret Santa page on your event page:
Change / Correct the participant's email
Send back an email

Still not received?
This is very rare but some anti-spam systems are particularly restrictive and require validation by Captcha (i.e., etc.):
*Request to add confirmation AT address in the contact white list of the recipient
Go through a less restrictive email address and then forward the email to the desired address (without trying to consult the draw, of course :)
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