I can't really understand that. Doflamingo seems to be able to use it on hundreds of civilians, but when it comes to Luffy and Law, he doesn't (or he can't). I myself believe it has something to do with Haki. But again, pirates with notable Haki and strength like Bellamy and Sanji got rekt and controlled over effortlessly by him.
I mean why bother with the bounty game in the first place? just manipulate the marines, the strawhats and their allies, and let them go all out fighting each other to the death.
Another thing is Doflamingo's "Black Knight; clone" . Is there a limit for the amount of which he can produce. I mean, those clones could pretty much finish all these small fries roaming in Dressrosa, and help his executives who seem to have fallen out already. (Was Doflamingo really confident that they could take on those Colosseum fighters, hadn't those four have a 100% of winning or some shit like that!)
I still believe though that Doflamingo has something up his sleeves, and he will show it eventually by the end of the arc. Tbh, I can't see Luffy and Law defeating him (not to mention Trebol), maybe if Sabo joins their fight, who knows?
sorry for my bad English. I would like to hear your thoughts! ^^