I've just completed the mission to retake the castle for the Minutemen. Preston was my follower before the mission, then left when it started to go to the meeting point.
After completing the mission he returned to Sanctuary and I can no longer speak to him, does anyone know if this is a bug, or there are subsequent quests I must complete to get him to follow me again?
Edit
I've since finished the old guns quest and I spoke to him to collect the reward but couldn't bring up any extra dialog with him.
Best Answer
There's a workaround in the wiki: (WARNING: Wiki is full of spoilers; I've copied the section below and censored spoilers)
I've read somewhere that all this faction code does is block companion capabilities, which makes little sense as the last line of dialogue for him is that he'll always have my back.
Anyway, you have two options (unless you play on PS4/Xbox One, in which case #1 is your only option):
I used the console command and I didn't see any side-effects, even a little further in the main quest. I still highly recommend doing a full save (manual one, not auto/quick) before doing this. You can also restart the game afterwards to ensure the game state is stable. (Though that's really not needed and mostly voodoo on my part)