While the Dragonflights are/were deemed/charged as guardians by the Titans, each of the separate dragon aspects/flights have their own ways/agendas (if you wanna call it that) that may or may not correspond with those of their fellow aspects/flights.
Hence the slow response that VeljkoM was mentioning among other things. To be honest they're just too divided by their own thinking/acting to even have an instant reply/response/action towards Azeroth's current/past/future doings/happenings. (Time-travel, yuck)
So shortly they were licking their wounds or/and pursued their perspectives disregarding mortal affairs like Scourge/Legion Invasion during the 3rd war and beyond.
A bunch of sleazy/fat bags, that's what I'd say.