Some of my subscribers in Klaviyo don't have a discount code generated in Personal Discount. Why is that?

The discount codes in emails sent from Klaviyo and any other email marketing provider, besides Mailchimp, are being generated at the moment when recipients open their emails. Additionally, external images have to be enabled and loaded in the email client.

You can check the contact profile in Klaviyo to see whether they received and opened the welcome email. Klaviyo relies on loading an external image to track email opens so this is a good test to check if a discount code has also been displayed.