Why wordpress doesn't generate thumbnails?

Wordpress doesn't generate thumbnails in my server. I don't know why. Old images' thumbnails get regenerated; however, the newly uploaded ones doesn't. I used Regenerate Thumbnails plugin to regenerate all the thumbnails. The new ones doesn't regenerate. It gives me error "The error message was: Unknown failure reason."

I have checked everything. I have GD enabled in php.ini and the directory is writable. What else should I check?