It was a conflict with the Emmet package.
I found out that this was a conflict using cmd + . keyboard shortcut
That let me know that in the file emmet.cson there was a conflict with ctrl-shift-m that is used to merge lines.
I don’t use that in emmet so I just commented it out (placed a # at the beginning of the line)
It still didn’t work.
I shut down atom and restarted and then ctrl+shift+m showed a preview of markdown.