Default Blog Page – 2 column grid not responsive

When you test out the layouts for blogs as follows, one is responsive (as in posts are single column and look good) and other not – see details below

DEFAULT BLOG PAGE with right sidebar, and a single column post with 2 column posts below
When reduced to the viewport of a cell phone, the 2 column posts do not transition into a single column format and they are squeezed into 2 columns – looks awful.

BLOG CLASSIC GRID
This has blog posts 3 across in a grid full width page (these are the same size as the 2 column posts above). When you reduce viewport to cell view, these DO form a single column and look good

QUESTION: Why is the 2 column layout of posts on the default blog page not changing into a single column for cell phones? How can this be fixed.