Twitter cards integration not working

Hi guys, I try usin WordPress SEO to activate my twitter cards. However they don´t seem to work.

My site is nicowegher.com I go to SEO/Social/Twitter
Select “Add Twitter card meta data”, then complete my twitter username with out the @ (nicowegher), select “Summary” as Twitter card type to use and save changes.

After doing this, I refresh my site and check the code, but there is nothing new in the head section. No twitter card meta data at all, therefore I can´t apply on twitter to validate the site.

Any help?

Thanks

https://wordpress.org/plugins/wordpress-seo/