I work with batiks a lot and I've never had a problem with stretching. In fact, that's why I work with them. I never starch batiks either. Were they cut on the straight of grain? Were they all the same type of batiks? I've found I can't mix ConnectingThreads batiks with others because CT batiks are a thinner weight; I noticed they didn't press as crisp or hold the triangle shape as well.
Guess I'm not much help--just offering some thoughts.