My new 64x64 skin was working fine until I disabled the individual parts in the skin customization menu to check how the bottom layer looked. When I re-enabled the hat/jacket/sleeves/legs they appeared completely transparent. I can tell that they are still there because there is an outline around each section, similar to when a block is highlighted. This highlight disappears when I disable the parts again, so I know Minecraft is trying to render it, but isn't. I've tried re-uploading it several times, even editing it slightly and renaming the file, and even uploading my old skin and then the new one again, but nothing seems to work.
SOLUTION: The unused white areas of the template must be set as a "transparent" color even though they are never rendered. It may simply require a single instance of transparency in the image to register as the 1.8 skin type. (mine did not, even in the higher body layers) but I'll leave it to someone else to test this out.
SOLUTION: The unused white areas of the template must be set as a "transparent" color even though they are never rendered. It may simply require a single instance of transparency in the image to register as the 1.8 skin type. (mine did not, even in the higher body layers) but I'll leave it to someone else to test this out.