I understand you're experiencing issues sending preview emails in Klaviyo. One common cause is reaching the preview email limit associated with your account's billing plan. Each plan has a specific limit on the number of preview emails you can send per billing cycle. For instance, free plans can send up to 100 preview emails per cycle, while paid plans have higher limits based on the number of profiles.

If you haven't exceeded your limit, another potential issue could be the formatting of the email addresses you're using to send previews. Ensure that when sending to multiple recipients, email addresses are separated by commas without extra spaces. Incorrect formatting can prevent preview emails from being sent successfully.

Additionally, it's advisable to check your spam or junk folders, as preview emails might be filtered there. If the problem persists, consider reaching out to Klaviyo's support team for further assistance.

For more detailed information on preview email limits and troubleshooting, you can refer to Klaviyo's official documentation.