ACPI has different sleep types. for WOL to work, you need either S1 or S3.
Basically what is happening, is if your device goes into "deep" sleep, ethernet interface is shut down and after a while, 30 minutes or so (depending on your device), ARP cache expires, and no devices on your network know how find your box now, since WOL works on MAC address basis.
I am not sure about your router, but on mine (WNDR-3700 with DD-WRT firmware) you can hardcode MAC and IP addresses, so the router knows where to send the "magic packet", as long as your box not in the "deep" sleep.
Apple AirPort routers have another way of dealing with it. They don't let your devices go into the deep sleep, by temporarily waking your devices from time to time.