It's made in Unity. I don't know what might be causing that.
Press S or the down arrow key to ground pound.
If that doesn't work, click on the game to make sure it has focus and try again.
If it still doesn't work, try unplugging any USB devices. Alternatively, try using a USB gamepad.
If it still doesn't work, it might be a bug.
Don't worry, you're not the only one who's had this issue. I've made some changes in the next update that I hope will help.