When a game was flat, there are questions that I ask to try to diagnose what went wrong. Essentially, I look at the tools the game gives us to make the game hum right along and shift from third to fourth and fifth gears and consider how they were used. Games don’t just have one tool and sometimes the use of tools isn’t the problem at all but it is a place to start thinking.
Burning Wheel: What were the players’ beliefs and what kind of artha hit the table?
Sorcerer: What were the kickers and what was written on the back of the character sheet.
I think Apocalypse World, Dungeon World and Sagas of the Icelanders might be, “What questions did you ask and what kind of answers did you get?”
A flat session might also have something to do with hard moves chosen on failed rolls and what is offered on the 7-9 rolls but asking the players solid questions and using the answers feels like an important detail that is often overlooked (or at least, under-discussed where I’m reading).
This isn’t to say that there aren’t tons of other mistakes that can be made with any of those games. I’m just talking about first thoughts when my or someone else’s flat session.
Githyanki Diaspora 
I tend to look at two things: energy at the table, and what generates momentum.
Flat games for my group usually are predictable ahead of time if we know how the game system works – we’re all tired, a little off. We’re getting better at just saying, “Hey, today it’s not going to happen”.
For a new game, we try it out and more often than not we end up looking at what it does/doesn’t do compared to Primetime Adventures. We tried something a couple of weeks ago and a lot of the problem was that the game doesn’t do a good job of setting up PC motivations/goals to scenes loop – the GM has to throw out a lot of ideas, the players pick up on one, the GM has to try to follow it, etc. all while improvising. A lot of work that wasn’t kicking off well but basically fell into that classic problem of traditional games where it takes 3-4 sessions to even find it’s feet.