If Masthead is not displaying on a page for you, please check this list.
There are a few places where Masthead will not work based on limitation in the SharePoint Platform. These are:
- System pages on Modern Sites (e.g. Settings pages)
- Classic Lists on Modern Sites (i.e. Lists that do not yet have a modern experience such as the Calendar views)
- Subsites on Modern Sites (While you can technically create subsites off of Modern sites, Microsoft advises against it. These sites will not render Masthead)