Up jumps and down places a box. However you have to jump then place boxes mid-air. And this is required to get through the game.
This wouldn't be a problem except for some reason the game won't let you press down while holding up, a thing you will be doing often *because you are jumping then placing boxes in mid-air*!
So you must jump, then quickly release up, and then press down. And this is awkward to say the least. I don't know why you programmed it like this, but that should be fixed. You shouldn't have to release up before pressing down.
I know it's not key ghosting because, one, the arrow keys don't ghost on this particular keyboard I'm using (at least if I only use the arrow keys), and two, you need at least three keys held down for key ghosting to come into play, not merely two.
There was a similar thing in someone else's game where you couldn't press up and down at the same time and when I brought it to his attention, he fixed it. Is this some insanely stupid default in Unity or something?