To keep your chart clean, it only shows these colored sessions on smaller timeframes (anything up to 30 minutes, plus any second-based charts). If you switch to hourly or higher (1H, 4H, Daily, etc.), the background shading is hidden.
You can set up alerts so that you’ll be notified the moment a new trading session starts. For example, if you enable the “Sydney Session Start” alert, you’ll get a pop-up or push notification (depending on your TradingView settings) whenever the Sydney session begins on your chart.