You can only get up to four AMD cards on Windows 7. You need to upgrade to Windows 10. There is supposedly a patch workaround to get more cards working with Windows 7, but I couldn't get it to work with the latest AMD drivers.
https://bitcointalk.org/index.php?topic=712228.0All of my 4+ card AMD rigs are working on Windows 7 with no problem for more than 3 years. Try running the mod with administrator priviledges, its pretty weird that it wont work for you.