First of all, sorry for my bad english.
I just had that problem yesterday, and I found the solution today, try to add your PC to the ARP table on the router.
The only difference is that it's the first time I try to use remote WOL on this router (I used to have a different ISP), so before the ARP modifications WOL from WAN did't work at all, but you say yours used to work and it suddenly stopped working... It would be helpful if you could share some more details (router brand and model, PC OS, etc.)