Server package: WOL policy not working

I have two servers at different sites, on one I have a scheduled WOL policy
working perfectly, but on another I just can't get it to work.

On the site that's not working, I can wake workstations directly via C1. The
WOL service lookup schedule is set to daily, all days, office hours, repeat
every minute, and the server package WOL policy is scheduled daily for a
known-good target list. I've stopped and retarted the WOL service at the
server console, and the console shows that the WOL service has started
successfully. However, no WOL packets are seen on the wire, and indeed none
are reported at the console.

Clue: I've just noticed that Remote Manager > Diagnose Server > Health
Monitor shows many failed logins per hour, for the 'user' "Server Package:
Wake on LAN". Huh?

Any suggestions as to where I go from here much appreciated!