It maybe that you don’t have this at the top of your child theme style.css:
@import url(“../twentytwelve/style.css”);
replace twentytwelve with your theme name, this should over-ride your parent style.ss
It should be placed directly after this (any twenty twelve text replace with your theme name):
/*
Theme Name: Twenty Twelve Child
Theme URI: http://example.com/
Description: Child theme for the Twenty Twelve theme
Author: Your name here
Author URI: http://example.com/about/
Template: twentytwelve
Version: 0.1.0
*/
