Hey,

well there are some specific points in the game where the doublejump does not work at all or just after you jumped with a normal jump before attempting to double jump. (on these points it sometimes look like a little lagg...maybe this is what you mean)

These places where you can't doublejump are in some stages right after a checkpoint. For example Level 14 (Dragoon Skyfort) after the dragon passage: You enter a checkpoint and then you have to do a single jump...then you can double jump. Without a normal jump right before youll never be able to doublejump here. There are more checkpoints like that in the entire game.

So all in all, the double jump mecanics are kind of messed up in this game and it needs some time until you don't get screwed by the bad programming :D