You used for your button flat design, which is basically a design, that usually either uses a strong shadow with sharp contures in a 45° angle (some shadows fade away in one directen but still have the strong contures at each shadow side...) or they don't use shadow at all.
Since this button is just really small it's difficult to make any shadow look reasonable (I won't say realistic, since it's not a photorealistic button ;) ).
An easy and fairly often used way to contrast the pressed button from the other is by making a darker version of it. (used: Layer style: Satin):
If you like you can maybe use a light gradient (used: gradient overlay: 45° linear burn):
I tried to play a bit with the button "wall". Since you can see the wall, there is a part that you then should not be able to see on the other side. But that just looks weird (Although I have to admit that the shadow is all wrong in this. But putting it on the other side didn't make it any better):
(this is just awful)
So I tried it more with the view on top but with the inner shadow in a 45° angle way and a bigger shadow. It doesn't look pushed in per se but still quite nice. And if you see the effect via mouse over I think it can work quite well:
Another top view with a gradient and conture (I used inner glow for the conture and inner shadow for the gradient) It doesn't look bad but may not fit to the flat design that well:
Another way to really have the push down effect while clicking or mouse over would be to give the original button a drop shadow (its more the outer button "wall") and have the original at the pressed button. Its a small detail but gives a nice effect while in motion, I imagine. (Here you have to keep in mind that the pressed button needs to be on the same baseline than the non pressed button. So the not pressed button is a little higher at the top):
In the end it's your decision to which you like better.