Talents,
If you had read the thread, you'll have seen that the scenarios where cargo are not shifting include those where is only one route open for given catchment areas on either side. The expected game behavior there should be actual demand increases to match potential (at least, that's how it has worked since the feature was introduced). I have done the research in my end from FRA - no other FRA catchment airports with routes to these places and these airport are so small that they do not intersect with any other airport.