When you create an auto email in Matrix, the recipient/Contact has to open the first (Welcome) email and click on the link within it in order to activate their Portal and receive any future auto emails. Sometimes email filtering can get in the way here- the recipient's email filter will think the Matrix email is suspicious and move it from the Inbox to the Junk/Spam folder, so the recipient may not actually ever see the email.
Matrix does allow you to resend the Welcome Email, in case your recipient (for whatever reason) did not see or open it when you first set up the auto email.
Go to My Matrix> Auto Emails and click on the drop down arrow on the left side of the desired Contact's row. Hint: you will also see a little green/white empty circle in their Status column, which indicates they have never clicked the link in the first email you sent them:
Click the Resend Welcome button to resend the Contact the initial/Welcome Email. After you send it, you may want to reach out to them separately and let them know that you have done so, and ask them to keep an eye out for the email (both in their Inbox and their Spam/Junk folder). Once they receive the email, they need to open it and click on the link to the listings in order to activate their Portal and start receiving their scheduled auto emails.