This happened to me today also: add the materials to the gun thingy and it starts up and no mobs spawn. Tried destroying the gun adn rebuilding it and still no mobs spawned, tried again and nothing again. Left the vault completely and tried in a new one and it happened again, so we gave up and did the Eoten vault instead.
This happened to me today also: add the materials to the gun thingy and it starts up and no mobs spawn. Tried destroying the gun adn rebuilding it and still no mobs spawned, tried again and nothing again. Left the vault completely and tried in a new one and it happened again, so we gave up and did the Eoten vault instead.