When you click down the button of sleep, only screen is off but the PC is still running. After rebooting, sleep will work.
Why? Because some process is running and preventing your PC from sleeping. After rebooting the PC, the process will be terminated so that you can sleep the PC.
We can use just one command to find out the process.
Open your CMD or Powershell, and input this command:
Well, we can find that a process named NeatDM.exe is preventing sleeping. It’s a download software. After closing it, I can sleep my PC at once.