Windows – My laptop will not sleep when I close the lid

I've tried to look into everything I know of but this has stymied me.

I have a Lenovo Thinkpad Edge 15" currently running Windows 7. The laptop will not enter sleep mode when I close the lid. When I close the lid, all it does is turn off the screen and lock my Windows session (requiring a password prompt when I open it again). But it does not enter sleep. It doesn't matter if I'm plugged into the wall or not.

I have tried:
Checking the power settings. I have it set to sleep when I close the laptop lid regardless of whether I'm on battery power or plugged in.

Checking Lenovo's power control panel (which should be reflected in Windows' own settings anyway).

Checking for anything that prevents it from sleeping after it failed to sleep.

Checking the BIOS (there was nothing indicating that sleep mode was disabled).

Trying to enter sleep using other methods:

• Windows will enter sleep automatically if I leave the laptop powered on and idle for a long time.
• Windows will enter sleep if I click the Sleep option in the start menu.

Closing the laptop lid when not logged in also resulted in the same behavior, so it doesn't matter if I'm logged in or not.

I also saw nothing in the Event Viewer indicating that any system errors occurred when it attempted to enter sleep mode.

What else can I try?

P.S.: Up until very recently, I had Debian installed on this machine. I wiped that and restored the same Windows 7 image I had made prior to installing Debian. On Debian, it was able to sleep the laptop by closing the lid. On the same Windows installation prior to me installing Debian, I was able to as well, from what I recall. This is the same Windows installation that came pre-loaded with the laptop. It is otherwise well maintained, as far as I can tell. I have been trying to keep it clean, as the laptop is 5 years old and can slow down significantly if I don't pay attention.