Single sign-on configured for your Aha! account (not the ideas portal) will only work for users that are part of one Aha! account.
Users registered to multiple accounts cannot be configured as single sign-on users because single sign-on is enabled for the specific account and then overwrites the user setting to have a standard Aha! username and password.
If you enable single sign-on for your account, any user that is not able to convert is likely associated with a secondary account. Often this is an expired trial.
If this happens, please contact firstname.lastname@example.org. We can help remove the user from their secondary account which will allow them to correctly convert over to single sign on.