Your CSS should not only contain
font-family: 'Source Sans Pro', sans-serif;
it should also have values for FONT-STYLE and FONT-WEIGHT:
font-family: 'Source Sans Pro', sans-serif;
font-weight: 900;
font-style: italic;
in case you use a font that contains those values like for example:
https://fonts.googleapis.com/css?family=Source+Sans+Pro:900italic