This can be caused by a few things:
- File Permissions
- Corrupt Install
- Incorrectly named theme folder/nested folder
- Domain mapping issues on subfolder installs
1. If you have a caching plugin installed, ensure it is setup to NOT cache for logged in users, and clear it before setting up the theme. If you have WP Super Cache specifically, try deactivating it
2. Try reinstalling the theme via FTP with a fresh download of the latest version. This ensures no files were damaged or incomplete during upload:
How to reinstall your theme to fix issues
3. Ensure your WordPress file permissions are correct. All folders should be 755 , and files 644 with the exception of the wp-content/uploads folder, which can be 777.
4. If you still have issues, ensure your site is configured properly to handle the way your URL is setup. By default, wordpress expects to be installed in a path like yourdomain.com/, not subdomain. yourdomain.com or something.com.yourdomain.com as currently. See the following for more information if you have WordPress on a subdomain or subfolder:
Giving WordPress Its Own Directory « WordPress Codex
Go to Settings > General and verify the URLs are correct. If your domain is http://www.yourdomain.com. the urls entered here must have the www. If the domain is setup as http://yourdomain.com, then ensure these urls do not have the www.