Why is my phone's display not getting off even after the "sleep time"?

We usually set screen timeout at some specific time (15 seconds, 30 seconds, etc.) for the screen to get off after inactivity. But my phone's display is not getting off even after the set screen timeout's time passes. E. g. If I've set the timeout at 30 seconds after Inactivity but the display continues even after 30 seconds of Inactivity. I can only turn off the display by the power button now.