Dumbledore and Grindelwald were romantically involved and in love with one another, so Dumbledore was always reluctant in stepping in himself to stop Grindelwald until he had no other choice. There is also a story (disagreement over "the greater good" ideal) when Dumbledore, his brother Aberforth (seen in Deathly Hallows and is also going to be in this third FB film), and Grindelwald where their relationship turns sour, resulting in a duel that kills Dumbledore/Aberforth's sister (Liana? Forget her name...) from a stray spell that strikes her.

(Dumbledore always believed it was his spell/his fault that his sister was killed, which was the fear he was experiencing in the Half-Blood Prince when he drank the water in the cave. And this is a big reason why Aberforth dislikes his brother.)

I imagine this third film will begin to get back on track with the main story towards the end. The second part above was covered pretty well in the Deathly Hallows book, but the film largely (almost entirely) excludes it. (You only see their sister in the painting that lets Harry and company into Hogwarts in the last film.)

Back to these films... The story taking place is currently dealing with the Credence storyline (which, to my knowledge, is original to these films) which I'm guessing gets resolved in this third film. I think this was just added so that more films could be made, honestly.. A fourth film should be plenty to wrap it all up though with the eventual Dumbledore/Grindelwald duel, so we'll see (this is how Dumbledore claims the Elder Wand). The only way I really see a fifth film is if the Credance storyline *isn't* resolved in the third film and it rolls over into the fourth film or something.