UIButton需要点击两次才能触发其动作

I have two round rect buttons. I have implemented its touch up inside action. It works fine but when my click second button just after clicking the first button, my first click has no effect. This is happening with both the buttons. I am actually trying to change background image of buttons on each click to get the effect of check box (checking/unchecking). Below is the code for for one of the buttons:I have two round rect buttons. I have implement