1) Follow-up rule must be active.
2) Our extension requires cron to be configured and constantly running in order to send emails. Please make sure Magento cron is configured and working on your store. Here you can learn more of Magento cron and how it should be configured.
3) By default, the extension requires cross-sell products to be set for your products in Magento. If they are missing, emails won't be sent out. In order to overcome it, please open the corresponding follow-up email template (go to System > Transactional Emails) for editing and find the line: handle="amfollowup_email_crosssel". Modify the line the following way: handle="amfollowup_email_quote", just like in the screenshot:
Save the template. Once done, the extension will no longer need cross-sell products to send emails.