The two most common causes for this are a simple typo when creating your MotorTrend login. In your mobile app double check that the email address is spelled correctly on the Account Information screen under settings (which may be a link inside the Notification Center).
The other common cause is that Apple's App store or Google Play store may not have synced with our systems. To force them to sync with our systems, log out of the app, uninstall the app, reinstall the app, and then tap on the Join option (not the "Sign In" option). You are not going to re-purchase the app, but signing in on this screen should cause the app store to identify your subscription and sync to the MotorTrend systems. If this doesn't work, please confirm your subscription shows up in the app store area for managing your subscriptions.