Systems Management

I have a lab full of GX620s that respond to WOL as such:

If I put the computer to sleep, then send WOL, it wakes.

If I have the system powered on, power it down, and then send WOL, it wakes.

If I shut the system down, wait for ~24 hours, and attempt WOL, it fails to wake.

I have the BIOS set to allow remote wake up. Have low power mode disabled. Set suspend mode to S1. Have the NIC configured in windows to "allow this device to wake computer" (which is necessary for at least the Optiplex 760s).

Does anybody else know what could be going on here? Something about the computer being shutdown for extended period of time makes it different from shutting down for a few seconds?

I do also have Windows configuration for the NIC to allow the system to shut down the device, which is necessary to set the "allow this device to wake computer" setting, but if I look at the NIC of the system it always looks the same, with link lights active.