4.6.1p2: touch or box collider

i have made a candy-crush-like game and port it to android. it can run now. but i got a little problem there. you see, when i (user) click a sprite ( i made its function as if it is an exit button) the quit application function doesn't do its job until at least i (user) clicked it for three times.

just fyi, i use box collider2d without any script for collider only OnMouseDown and input getbuttondown. and the problem also affect my array of item sprite that supposed to be clicked and swap by user.

why? does anyone know? must i change to touch input?