The ground game in general and the lack of force tech are something that have changed quite a lot in Ultimate and many of the top players and veterans say it'been getting worse since then.
Personally, I can't really relate because I did not play former doa games and did not care as beginner in 5 vanilla, but the main reason might be that it is quite difficult to hit people on the ground with some characters.
In general, you should be able to hit someone after grounding them with a so called hard knockdown. This is a knockdown where the opponent is not able to tech roll or use a wakeup-kick immediatly and stays vulnerable on ground.
Depending on the moves you use, it is up to you to find out what can be used as a hard knockdown. I don't know a clear rule, some characters have more of these moves as others and there is often a special moment where you need to land it.
For your question, I have to say first that I'm also not aware of all things about the ground game.