Are you using the wp-discourse plugin with WordPress as the SSO provider for Discourse?
If so, it doesn’t create accounts on WordPress for users that existed on Discourse prior to enabling the SSO functionality. If this only affects a few users, the easiest solution would be to ask those users to sign up on the WordPress site using the same email address as they are using on Discourse. You could also do this yourself manually in the WordPress dashboard.
If it affects a lot of users, I’m not sure right now what the best approach would be. I’m surprised this hasn’t come up before. I’ll see if I can think of anything.
Edit: The easiest/safest thing I can think of would be to create a banner topic on your Discourse forum, that explains how the login process has changed and provides a link to the registration page on your WordPress site.
It would also be possible to write a script on that would run on WordPress to sync users automatically, but it would take a bit of work to get it right.