Sometimes, even when implementing responsive CSS using a media query, the website doesn’t become responsive and shows the same nonresponsive design on every device.
It’s maybe because the viewport meta tag is missing in your HTML file. Just set the viewport and default zoom on meta as shown below in the HTML head section.
<meta name="viewport" content="width=device-width, initial-scale=1.0">
Then, your media query CSS starts working and the website becomes responsive.