Thanks for writing in!
Upon checking into your website, I’ve noticed instead of to be chosen ‘Lato’ font for post headings, you actually have chose ‘Cantarell’, which don’t allow the font weight more thinner then 400 (see: http://prntscr.com/9yuu2s). Please chose the ‘Lato’ font under Appearance -> Theme Options -> Typography -> Post Title & Content. As per shown here (see: http://prntscr.com/9yuwpg).
On a side note, please remove all custom code related post headings font you’ve added to your custom Css field, so that the theme’s default styling doesn’t get affect.
Hope this helps.