Shih Tzu Q&A
Try a better quality food. You can also try adding Pineapple to the food as it will make the waste taste bad.
Our American Bulldog did the same thing and the vet told us it was because his food wasn't fully digested and he could still smell the food in his waste. Gross, I know, I think he's grown out of that stage though. He's now a year old and it was about 4 or five months ago when he finally stopped.