I have the same issue. Upgraded one of the phones on my account from iPhone 5 to iPhone 6. I can still track the iPhone 5's on my account but cannot track the iPhone 6(previoulsy a iPhone 5 and tracked using Family Map without any issue).
I've tried all the recommendations posted but no success. App is fine, iPhone 6 is registered. AT&T what's the issue? Is my account going to credited every month until this issue is resolved?