Often times even if we have setup our media queries correctly, the site still isn’t responsive to those queries.
You need to set a viewport in your header.php to fix this. More often than not I find that this affects macs instead of PCs. but it’s still a good idea to add this no matter the case.
In your site <head> add this code:
<meta name="viewport" content="width=device-width, initial-scale=1">
And it should fix the problem. This is telling the site to be aware of the device width that is viewing the site.