UpSnap wol only few seconds after turning off a pc

[lucasca-maker]

Hello all!
I installed UpSnap on my qnap TS-431k nas and I would like to wol the pc's connected to the nas.
My problem is that it turns on the pc's only if they have been recelently (10 seconds) turned off. I can always see the state of the pc's (if the are on or off) from UpSnap

Thanks for the help
Luca

P.S.
if I try to send the magic packet from another pc inside the same network it akways work