Housebreaking a 1 year old

Question
I recently bought a 1 year old Affin Pinscher from a shelter.  He is a male named Clyde.  After intitally seeming as though he was housebroken (there is a dog door to go in and out of, he got the hang of in about a week) he now goes to the bathroom in the house about once a day.  What changed or what can I do to housebreak him again.

Answer
He is neutered as most dogs from the shelter?  It could still be a leadership thing. The key to most behavior problems is approaching things using the dog's natural instincts.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm

Otherwise this could be tough to fix if you are leaving him all day by himself.  Even if you careful reward him for going out when you are around, when you aren't around is a completely different thing.  I have never been a big fan of doggy doors. Having a job and a dog too is tough.  Most of the time my wife's and my work schedules have been such that the dog was never at home by itself too long or we could even take the dog with us.  I like leaving the dog in the  house in a crate and finding a way to give it a mid day break.  Another excellent alternative if possible is doggy day care.   

One last straw to grasp, a vet visit.