LTI Configuration XML / Authorization Fail

[DLilly]

I am working on integrating external LTI and have run into an 'authorization failed', error anytime the content linked to is prompted to launch. For context it is the HMH set of LTI integrations I have been working to configure via XML. I've double checked that I've entered the Key and Secret correctly and made sure everything was in plaintext. HMH has verified the validity of the XML/Key/secret. Don't know where to go for troubleshooting next. I've installed it at the root account level.

Thanks,
Danny