If you're doing good on direct offers, why are you playing the incent offers game? Instead of linking them to download a game which most incent offers do, why don't you use email submit travel offers?
The reason why it works on some niche and not others, is because of what offers are being shown even if it is incent. A travel niche, and asking you to download clash of kings, is just strange.
That is why for incent, as you have mentioned, is a volume game.