By and large, it isn't a manufacturing quality issue. There are service issues and sometimes the battery is simply outdated and dead, despite FAA mandated inspections. Activation is by a g switch in a crash of an aircraft. That can be a rather violent sort of experience. Sometimes either the ELT or the Gz switch is simply destroyed, even though tested to 40Gz and in fire and all the rest. The ELT is connected to an antenna. If that cable is severed, it doesn't work so well. If the antenna is broken off, it also sort of doesn't work so well. If the aircraft ends up with the antenna under the wreckage, it doesn't transmit to the satellites. There's more ways for an ELT to fail than you can shake a stick at. The ELTs themselves are pretty darn robust and reliable, but they are just a part of the system and any single piece in the chain can cause a failure.