Windows 7 Client Cannot Wake Server

Hi - recently I updated Lights-Out on my WHS2011 to version 1.5.5.2136.

Since then, booting up my Windows 7 desktop does not wake the server any more. My Win XP laptop still wakes the server fine.

Both computers have the latest client installed, both are set in L-O to 'Automatically wake server on boot or resume'.

My various IP devices still work OK with L-O, and I can manually wake the server with a WOL app on my smartphone.

Can anyone suggest what's happening here? My W7 desktop is the main computer, and I want it to wake the server on boot (this used to work great before the L-O update).

thanks,

Simon