Does yogurt "wear out"?

OK, so I used Stonyfield Farms yogurt as a starter for a batch of yogurt that came out fine. Then I used some of that batch to make the next one; it came out not quite smooth, but leaning toward ricotta texture, & was a bit more tangy. Then I tried to use some of that batch to make more… nothing happened. I scalded & cooled the milk as usual, stirred in the starter, left it in a warm oven as usual, but after 5 hours all I had was warm milk with a slight yogurt flavor. Anybody know what went wrong?