Some quick things I'd try:

1. Have you logged inside the try block to check that it's actually attempting the call?

2. If it's not logging this out do you need to run the code snippet inside the runTransformScript function like:

(function runTransformScript(source, map, log, target /*undefined onStart*/ ) {
	// insert the Sub flow code snippet here
})(source, map, log, target);

3. Is the Sub flow active and published?