I think (unconfirmed) that there is only a 90% guarantee of getting NTG on a 5 star attack, and you get more/chances are increased by how much time is left on the clock when you five star. I don't think it can be any more than 90% because of the additional 10% chance available from 5* in the university. i notice I rarely don't get NTG from bases I five star with time left on the clock.
I actually like this element of the game, though I know many in my alliance still struggle in getting the NTGs ready for every war but it means that if you have high level coalitions you have to strategically decide which coalitions to upgrade, when to use your best ones, etc. I think the UN has helped atomic age players with this too, especially those advanced players who don't have time to make so many attacks in one day.