If you open a new shell, you have to tell your docker client how to connect to the docker daemon that is running in the virtualbox VM.
If you open the quickstart terminal, it runs a command that sets the shell environment up so that the docker client knows how to connect.
If you open a new shell, you have to do that step on your own. For windows, you have to specify your shell type. For cmd, run the following and then paste the output back into your cmd shell:
docker-machine env --shell cmd default
or for powershell, just run the following:
docker-machine env --shell powershell default | Invoke-Expression