Bootstrap toggle navigation bar not working in Mobile devices

We used bootstrap toggle/expansion navigation bar in our webpage and it was working well in all desktop browsers and also there is no issue even if we reduce browser size to mobile width.

But while we check in real mobile device browsers, the navigation bar not getting to toggle mode and still looks expanded like seeing in desktop browsers.

The problem was we missed to add the "viewport" tag in the html. :-)

<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">

we just added the viewport tage and now everything works perfect :-)