Here is the capx for the non working touch sprites:https://www.dropbox.com/s/bmjya8iefnjtjmk/alphaButtonSpriteFail.capx
It only works if you click on the + or - pixels of the sprite which have the alpha channel set to 255.
Any thoughts to why its not working appreciated.
Winks idea of using a hidden object underneath is probably the only workaround, but as I move this panel left and right with swipe, I can never get the objects to light up properly after (and you cant mix text and sprites in the same family). I will try pinning the invisible button to the non working button.
Its a bit ironic that an invisible object will catch touch events, but a visible one wont.