We had exactly the same problem with Louie, New neighbors move in and anytime they were out he would run to the fence and bark non stop. Bring him in and he would sit at the doorwall and bark in the house. Which was weird cause Louie is not a barker. So I would go out with him and follow him to the fence tell him " no bark It's ok no bark" and pick him up and start talking to the neighbors. After a few days of this he just stop, and likes them now.I think he thought they were not supposed to be their and he was on duty patrol
. or he just had to get use to them. Good Luck!! I know it can be frustrating.