Double-check over your coding, making sure everything's the same in response to your files in the .material file.
Also make sure when exporting, your username is typed in exactly like it was when you registered.
For example, mine's CloudFish, so I'd have to type in that, and couldn't use Cloudfish, otherwise it'd export white.
As far as exporting problems, check codes, .material file, and also try closing and reopening the game.