Hi,
You will need to load the content of the parent editable region on the master page to the editable regions on the sub pages. You can take inspiration in the code below:
MainContentText2.LoadContent(CMS.PortalEngine.PageInfoProvider.GetPageInfo(CMSContext.CurrentSiteName, "/", "en-us", null, true, CMS.DataEngine.ConnectionHelper.GetConnection()));
You will need to replace MainContentText2 with real ID of your editable region control.
Best Regards,
Juraj Ondrus