This commonly happens when the Customer Buys and Customer Gets section are mixed up, or you've assigned the 'free gift' product into the both sections.
Check that your 'Customer Buys' section is set to 'All Products'
Your Gift product is then setup in the section below called 'Customer Gets'.
Here is where you assign the gift product:
If you're trying to test the offer and the progress bar isn't moving, this tends to be the main reason why it happens!
Get in touch if these settings are in the right place and it's still not working.