You must give temporary vision of some sort to the player you want to beacon to flash for in order to get it to flash. The easiest way is just to create an observer over the beacon for the players you want the flash to appear to and then remove it quickly (around 500ms is the minimum for 100% guarantee).
The other way would be to give the beacon to players whom you want the beacon to flash to.
Either way works well, but the topmost works the best.