Ah, I think I understand! If you're throwing the dart, the aiming does gets a little wonky. I've had better success with impaling it onto the screen like a nail and releasing it. The dart is supposed to create fire in 3 blocks - at the point of contact and 2 tiles on either side.
I read that you played all 90 games over the weekend so I can't in good conscience ask you to strain your eyes any further this soon :p