While working the Minutemen missions after taking the castle back there is an assigned NPC that works the radio. It is with this radio that you are able to deploy artillery with your smoke grenades. At some point while continuing on with other missions the NPC that sits in the chair in the middle of the castle running the radio gets out of the chair. Once he does he does not return to the chair but instead says that the array needs to be powered. At this point no amount of power will bring him back to the chair there is no command that will bring him back to the chair. This effectively renders the castle and artillery unusable for the rest of the game.
There are several theories as to why this happened the most prominent is that he leaves the chair to help defend the castle during a raid. Regardless this is a major issue that needs to be addressed in the next update