android:drawablePadding will only create a padding gap between the text and the drawable if the button is small enough to squish the 2 together. If your button is wider than the combined width (for drawableLeft/drawableRight) or height (for drawableTop/drawableBottom) then drawablePadding doesn't do anything.
I'm struggling with this right now as well. My buttons are quite wide, and the icon is hanging on the left edge of the button and the text is centered in the middle. My only way to get around this for now has been to bake in a margin on the drawable by adding blank pixels to the left edge of the canvas with photoshop. Not ideal, and not really recommended either. But thats my stop-gap solution for now, short of rebuilding TextView/Button.