Joey, at 5 1/2 months, was trained to go on trays also. I know he knows to go on them, but all of a sudden he stopped. We have been taking him outside regularly and he's doing real well on that, but when he needs to go in the house he won't use the trays. I tried taking a pad out of the tray, and he will use that occasionally, but still has "accidents". I noticed that you say you use newspaper; Joey would never go if there is any colored ink on the paper, only black and white. I often put newspaper on top of pee pad because he would only use pee pad once otherwise.
I'm sure the freezing cold has something to do with it. I think dogs really prefer going outside, but it's like once I started outside training; the pee pads just weren't good enough for him.
Nancy