If your mobile screen isn't rotating, ensure to check whether if your phone screen rotation feature is enabled.
For Android Devices
Open your device's Settings app.
Tap Accessibility.
Toggle on Auto-rotate screen.
Samsung Devices
Swipe down from the top of the screen to open the Notification Panel.
Tap the Auto-rotate icon.
To lock the orientation (Portrait or Landscape), tap the icon again until it shows the Lock icon.
For iPhone Device
Swipe down from the top-right corner to open the Control Center.
Locate the Portrait Orientation Lock button.
Tap it to ensure the lock is OFF.
Turn your iPhone sideways to test.
For iPad Device
Open the Control Center by swiping down from the top-right corner.
Ensure Rotation Lock is OFF.
For older iPad models, check the Side Switch. You can configure this in Settings > General > Use Side Switch To.
Turn the iPad sideways. Note that some apps, like Safari or Messages, are best for testing landscape mode.
If you have an older iPad, check to see if it has a side switch
If you don't see the Rotation Lock button, check to see if your iPad has a side switch. If you have an older iPad with a side switch, you can set the side switch to work as a rotation lock or mute switch.
Go to Settings > General. Under Use Side Switch To, select the option that you want.
Important Note on App Support
Not all apps support screen rotation.
If some apps rotate but others do not, the app may have been built by the developer to stay in one orientation.