Gary does the same thing! We have always taken him out front to do his business. When we put him out back, he absolutely does not go on his own. If Gary is out back by himself, I usually am in the kitchen so I can keep an eye on him from the window (yeah, my pup at 7 months still tries to eat things that he shouldn't) but he never goes potty.
I think they don't do it because they are young and still exploring the exciting back yard, so they forget about pottying and get caught up with using their little noses. They go out front so easily because from day 1 they have been going out there so every square inch of the front yard has already been investigated
