Request Help: Sprite Editor not Automatically Slicing Sprite Sheet

I added an alpha channel to the sprite sheet from the URL at the end(Unity is not letting me attach my current picture) and saved it as a .psd. However, Unity is still not letting me do automatic slicing; when, I try it and click 'slice' it does not do anything. I was wondering if you might have an idea why this is happening and what I can do to change it. I was wondering if it might have to do with the letters, but I would think it would give me weird cuts for them and not just prevent the whole thing from being cut. Thank you for any help. I am brand new at photo editing so my apologies here. This is the link to the sheet: http://www.spriters-resource.com/game_boy_advance/mlss/sheet/7453/