Here's the list of possible sources of problems:
- Magento cache (or external cache) was not flushed after installation of the theme.
- You can see all sorts of error messages if the Compilation mode was not disabled before installation.
- Some of the theme files were not uploaded or are broken. Re-upload original theme files to your server (overwrite existing files) and flush the cache.
- File permissions issue – refer to file permissions for more details.
- You have some third-party extensions which change/break theme's default behavior or cause some conflicts. Disable all extensions and flush the cache, then enable extensions one by one to check which one is causing problems. Refer to Thirdparty extensions for more details.
- You edited theme files and accidentally removed or changed something in the files. Re-upload original theme files to your server (overwrite existing files) and flush the cache.
- You have customized the theme by creating a custom sub-theme and your modifications override or break the default behavior of the theme.
- You have some elements that left from your previous theme (i.e. in Magento's "base" theme in app/design/frontend/base/) and that breaks the new theme.
Also check your log files in var/log/ (log settings should be enabled in System > Config > Developer) for more information about possible errors.