I find there are many issues with using Master Pages in Sitefinity. One of the biggest is deriving master pages from other master pages.
When you do this, you may be tempted to add Sitefinity controls to the parent master page, and expect them to show up in the child master page.
For instance, you may add a Generic Content Control to the header or footer of the parent and expect to see them in the children.
This will NOT work, and you'll find that Sitefinity removes all Sitefinity controls from the parent when it renders the child.
This will work as desired if you use Sitefinity Templates, which is the right way to go.
If you absolutely have to use Master Pages, you can manually add Sitefinity controls (or in the case of Generic Shared Content) to the child Master Page at run time.