Post Registration Thank You Email not firing!

Our Post Registration Thank You Email is not firing when you complete the program webform and have downloaded the offer on the post registration thank you web-page.

We have not launched the program (in a Static Campaign) and are simply testing our configuration.

The Initial Email and Repeat Email are sending, the Registration Form and Post-Registration page are working as expected. On the webform the prospect completes an Email attribute and the contact profile of an existing contact is also completed (with valid email addresses).

Just the Thank you email is not firing.

Any ideas and advice would be greatly appreciated