Yes there has been instances where the beacons signals didnt get received and no one was dispatched. These are radio transmitters, not mirracle devices as some people think they are. A fisherman down here a few years back set one going at the base of a cliff and bagged the rescue services for not comming all over the media. He should have learned to use the device first before putting all his trust in it.
The older analogue type can also fail if multiple transmitters are going at the same time. The newer 406Mhz digital beacons have solved this problem.