Does anyone know why this could be?

Hi all :) --I'm new here and it seemed to me that you all have such good advice that I thought I would join and see if anyone can help. Actually I have 3 dogs, all with various issues of their own, but I will start with my Dukie today. We moved about 8 months ago into a new house. There is only really a side yard, about 15ft x 30ft I would say. From day 1, this dog would not go potty in that yard. He will hold it until we take him in the front yard where he will go no problem. There is no issue of grass vs. rock as we have both. I brought the dog door from the old house that he is used to using. The only thing I can come up with is that the houses are 2 story and the house next door is really close and he just doesn't feel like he is outside in the side yard and therefore will not go. I have tried EVERYTHING. Special sprays and enticing smelling things from the petstore, taking him in the yard on his leash, actually bringing his poo from the front and leaving it in the yard, having the neighbors dogs 'go' in our yard with the hopes that Dukie's male instincts will take over, etc. The only good thing is that he absolutely will not go in the house, he will hold it FOREVER, but every time I leave the house, I'm counting the hours and worrying because I just KNOW that dog has to go potty and is waiting for me. We have 2 other female dogs who will go in the yard no problem, but even that doesn't do it for him :confused:. I HAVE to get this dog in that yard as the neighbors are not happy being continually barked at if they happen to come out of their houses while he is there, (another issue for another day), and I am pretty sure our Homeowners Association is going to have something to say about all of this when one of the neighbors finally has had enough or God forbid I miss a piece of poo. I certainly appreciate any advice and thank you much!!