@media query not working under certain screen size

Issue: CSS @media query are not working under a certain width (in my case it was under about 900px).

@media screen and (max-width: 600px) {
  body {
    background-color: olive;
    color: white;
  }

Solution: check that you have set the view port in your page head meta:

<meta name="viewport" content="width=device-width, initial-scale=1.0">