What breed is this dog? HOw long have you owned him for? Is he an only dog? If not how do you know it is him?

Are there any changes at all in any of his surroundings etc.? Like his food, new home, new addition to the family, new carpet, change of schedule, new medications etc., has another animal used that spot as a potty before?
If the answer to any of those is yes than that may be your problem.

If the answer to that is no then I would clean that area really well with a product used for that purpose like natures miracle etc. If you have already used an amonia based cleaner on that are then it may be worthless & he will still smell it there so the training may not work.
Then work on re-training him just as if he was a puppy again. Keep him away from that room/area when he is not supervised, a gate or crate or keep him in a seperate room etc. When you let him out are you out there with him? If not than I suggest you start so you can see if he actually goes or not. Give him a potty command & when he obeys PRAISE, PRAISE, PRAISE!
Also maybe a vet visit just in case, sometimes worms may cause this behavior or possible colon cancer. Also if he is not neutered get that done as it will help prevent colon cancer & other marking behaviors.
Maybe he is not getting enough excersize as he used to or as much as he needs then he may be doing it to get "even" with you for not providing him with the proper care.
Has his behavior changed in any other way besides this?
Maybe something is scaring him at night?
Maybe try feeding him earlier in the evening