The first thing I did was torque the case a little by twisting to make sure everything was sitting properly, this improved the button a little but not enough for my liking ymmv.
My next step was using an exacto knife to remove a very small bit of the height of the button on the inside of outer case and try again. After several times through this process I got the button just how I liked it and used a file to smooth out the surface. Be careful you don't shave off too much material and ruin the case, the other thing to be aware is no matter how much you shave you will never get the clicky feel you get with the volume buttons, it's just not that kind of button.
Alternatively you can ask zerolemon for a replacement and they will likely send you a new TPU outer case, however it is just as likely you will get another one that behaves the same.