I had similar difficulty with Maggy (Shih Tzu/Lhasa Apso mix). My problem is that I paper trained her during a wonderful Chicago winter (8 week old puppy and -30 degree temps don't mix) and had a hard time with the transition of getting her to go outside once the weather warmed up.

The thing that worked for me is that I bought some of that special enzymic cleaner from the pet store that removed the odor, then I put her food in the spot where she always went. She hasn't gone there since.

Good luck!