Make sure your quality settings don't force half-size textures or anything silly like that. You can also try to use a different packer (your checkbox is set to Unity, you can try a custom one). Also note that a 512x512 texture is a bit on the large side to be added to an atlas. You can keep it as a separate texture, especially considering that its dimensions are a power of two already. Atlases are ideal for packing lots of small elements together in order to reduce draw calls.
