Is there content referenced for each course object?
Is the content referenced in the object published?
Was a course object recently deleted?
Is the course object set to visible in the course outline?
Is there a payment object on the course? Was it added or made required recently? If so, learners could be blocked if they had enrolled before the payment object was added/made required. The payment object may be manually completed by editing the enrollment and marking the Payment object as completed.
If the learner navigated to the course object via a direct link was the link that the user was given a link that enrolled users have access to? For providing learners with direct links to course objects see Deeplinks to Objects.
For sites using different domains to access Learning Groups the site/learning group URL will have to be hard coded.
A learner is receiving a 'Your enrollment has expired' message when trying to access a course.