Why does cloning a private git repository via Drush make file fail using SSH authentication?
Using modules and not theme_name may resolve this.

projects[modules][type] = modules
projects[modules][download][type] = git
projects[modules][download][url] = git@bitbucket.org:company/repo.git
projects[modules][download][branch] = master


Here are a couple of useful links: 

https://groups.drupal.org/node/194303#comment-918073
http://www.systemseed.com/blog/drush-make-private-git-repository-github