This is a result of django-compressor/django-compressor#226. The symptom is that any assets referenced via "url(...)" in a css/sass file will not resolve correctly when running in DEBUG=True.
A clumsy workaround is to symlink "img" and "fonts" static directories into site_static/CACHE:
cd site_static/CACHE/
ln -s ../../static/img .
ln -s ../../static/fonts .
This problem should not exist when you're in production mode.