I started having the same problem as well and called in a trainer. We started the program just a few days ago but basically we're going back to square one. Basic heeling training and lots of focus training under as little distraction as possible. Then we can build it up again. Basically while on walks I keep the focus on me and I will do lots of turns and corners so he HAS to pay attention to me. If I see a distraction like a dog I will avoid it because he's just not ready for that yet.
Like I said it's been only a few days but there is already some improvement