There may be several causes, including setting in the BIOS that need to be tweaked, but every time I have had issues with WoL, it has been due to a setting in Windows.
You might know already that to get WoL working, you have to go into the network adapter configuration and set "Wake on Magic Packet" to Enabled, and you might also need to set "Wake on Link Settings" to Forced (not sure). Finally, you probably need to tweak the Power Management tab to check the "Allow the computer to turn off this device to save power" and the other checkboxes in here (actually, on the contrary, I actually had to uncheck this checkbox, which grays out the other two).
But what you may not know is that you have to turn off fast startup. Windows must put the computer in a special sleep state with fast startup, one that does not allow WoL to happen. Just guessing. In any case, to turn off fast startup go to Control Panel -> Hardware and Sound -> Power Options, click on "Change plan settings", then "Change advanced power settings", then expand "Sleep" and "Allow hybrid sleep" and set it to Off.
Anyway, that's what I had to do to get WoL working.