Okay. What did you do to piss Preston off? Are you an addict who killed Mama Murphy and goes around wantonly stealing, being selfish and just being a general jacka**?
That would explain it. Preston is such a goody goody.
I think I read something in the guide yesterday about having to complete something at the Castle before he'll talk to you again. Sorry, I don't have it on me. Maybe you missed something?
Maybe you want to look at a walkthrough of that quest? I'm sure it's mentioned. I'll look as well.
Got this from reddit and I think it's what I read last night. Check the generators and be sure they're working.
https://www.reddit.com/r/fo4/comments/3sdvir/preston_garvey_bug/
apparently it's a known bug.