Have a look through this topic to see if your SSO code is doing everything it needs to: Official Single-Sign-On for Discourse (sso).
If you enable the verbose sso logging site setting, additional details about the SSO login requests will be displayed in your site’s error logs. To view your error logs, go to Admin / Logs and click the “Error Logs” entry from the admin navigation menu. The relevant log entries will start with the text “Verbose SSO log.” If you see errors in the logs that are unclear, feel free to post them here.