Hate to drag principles of physics into this, but setting books on fire is really the same challenge of setting a log on fire: you can't, unless you prepare kindling, as any Boy Scout knows (maybe that's why JWs don't want their kids to join: it blows one of their urban legends out of the water). Same with cardboard: you need to tear it up a bit at the ignition point, in order to get it started.
The same with gas: people can over-soak the object, depriving it of oxygen, a required element in the fire triangle. Vapors are much more explosive/combustible, but people think that gasoline is combustible, so they oversoak the object, and end up extinguishing the fire by cutting off oxygen. Note: do NOT attempt to put out a fire with gasoline! It would be stoopid (sic).
So the people weren't lying, just don't know how to start a fire (or in their haste to counter Satan, maybe thought Jehovah would just help them to over-ride principles of physics). Most of us don't have real-world experience burning books, but that should be a clue: anyone ignorant enough to want to burn books is also not a likely to have read many of the ones that discuss physics. ;)