Preston Garvey will no longer join me as a companion after completing the first mission at the Castle. He still talks to me, and gives me quests, there is no conversation menu for him any longer in order to get him to join me.

Same here. He still gives quests and ends quests, but no dialogue tree.

Same i cant get him as my companion anymore, i have done nothing to offend him and have completed all minutemen and sanctuary quests.

I'm also having this issue. Does anyone know of any solutions for this problem?