The most likely cause of the Thank You page not being displayed is due to the security set on the Survey in Luminate Online. Secure surveys cannot be embedded.
  1. Login into Luminate Online
  2. Navigate to Content - Surveys
  3. Choose the survey in question
  4. Click Edit
  5. On Step 9. - Survey Security, ensure the box is unchecked
  6. Click Save
  7. Refresh the CMS page, fill out the survey and click submit. It should now redirect you to the Thank You page.