WM_SYSCOMMAND SC_SCREENSAVE doesn't work anymore for activating the screen saver?

I use HoeKey as my hotkey program in Windows 7. One hotkey is supposed to activate the screen saver:

^`=Wait|1000
=Msg|0|274|61760|0      ; start screensaver

This waits one second to avoid the key release event, and then sends a WM_SYSCOMMAND SC_SCREENSAVE message. It failed a few times in the past, but it generally worked. Now it fails all the time. I wonder why?

This doesn't seem to be a HoeKey problem, because a trivial C program can't activate a screen saver this way either. However, SC_MONITORPOWER always works in both HoeKey and the C program, shutting off the monitor when lParam is 2.

In the past when it failed I thought it was due to the adaptive screen saver timeout, but disabling that doesn't help.