Bug With Button.

In the image shown below, there's a button and behind the 'block of marble' there's a command block with two commands listed. First of all, I'll explain how I came across this bug. I was creating a "Find The Button" Game. When I was creating the commands to get sent to the other level and the Title screen command. I clicked the button which was placed under a slab, it worked of course. Once I clicked the button again(I always test commands two) it didn't regsiter the click for the button to be pressed down. So, I clicked the button again thinking i misclicked, but still it didn't work. I'm pretty sure the click doesn't regsiter because the button was under a slab, also i tested the other buttons which was placed differently. Thanks for taking your time reading allat.