Diagnosis
The monitor is not receiving the correct wake-up signal from the PC. This is usually a power management or cable handshake issue.
How to Fix It
Follow this checklist. One: On your Windows PC, open Device Manager. Find 'Monitors' and expand it. Right-click 'Generic PnP Monitor' (or the Dell monitor if listed), select Properties, go to the Power Management tab. Uncheck 'Allow the computer to turn off this device to save power'. Two: In Windows Power & Sleep settings, ensure 'Screen and sleep' timers are longer than your test interval. Three: Power cycle the monitor and PC.
If That Didn't Work
The DisplayPort cable may not support the necessary sideband signals for wake-up. Replace the cable. As a diagnostic test, use an HDMI cable. If wake-from-sleep works over HDMI, the DP cable is the confirmed culprit.
Heads Up
If you are using a desktop PC with multiple monitors, ensure the S2421HS is set as the primary display in Windows display settings. Some systems only send the wake signal to the primary monitor.