To render the content, the template must include {{ .Content }}.
Templates are either single page page templates (used to render foo.md or index.md) or list templates (used to render _index.md and top level sections).
The template selected is based on defined lookup order.
Simply specifying a different layout value in front matter will only work if the corresponding template can be found in the lookup order. This comment might help you understand how to specify a particular template.