I have a file share set up on a Windows Server 2008 R2 server I have a security group called "Allow file access" (not its real name!) which include a bunch of users and also "Domain Admins"
I have a single permission set up on the share to grant read/write access to "Allow file access" group. And also the security settings for the folder grant full control to "Allow file acess" ND Domain Admins
However, when logged on (as a Domain Admin) I get a Permission Denied error when trying to access the share.
If I explicitly add "Domain Admins" to the list of share permissions I am allowed access.
Am I not allowed to nest Domain Admins in other groups?
Log out and back in again to get the new security token.
(FYI, I tested your exact scenario and this was the issue for me, so I'm betting same for you)