Theme CSS not included if subtheme used

Am I doing something wrong or is it a known issue that when using a sub theme the parent theme's css and js doen't get included?
I tried all possible settings of core library, but no luck. It seems to only load css/js that comes directly from a theme's .info file, not the ones of its parent theme.

I tried it with subthemes of omega and twitter_bootstrap. When I switched back to the parent theme all styles got included correctly.

I'd be glad to find a solution to this, as it doen't make sense for me not using subthemes.