Only some flatbread recipes are expected to "fully inflate" during cooking. Mexican flour tortillas, in particular, will have bubbles up to around 2", but shouldn't puff up into a balloon. So "what might cause it" is you doing things correctly.
In order to fully inflate, the inside of the flatbread needs to be weak enough when the outside gels and dries, such that the force of the steam can rip the inner crumb but not the outer crust. That means the hydration can't be too low, the flatbread can't be too thin, and the temperature can't be too low. Full inflation is unlikely to occur when cooking on a griddle (with no radiant heat from above) since the top won't be cooked. You might be able to get a griddle flatbread to inflate by cooking for a short period at a very high heat on the first side and then flipping, but I haven't tried this.