When you assign a Custom Template to a specific menu item using the Menu Style option when editing a menu item and its not working, then try the following steps to solve the problem:
- Turn off SEO in your Joomla! Configuration.
- If the Templates are changing after this change check to see if you have the SuperTable Plugin enabled.
- Disable the SuperTable Plugin
- Turn on SEO in your Joomla! Configuration
- Templates should now switch without any problems.