I am enrolled in their(edX) probability course, but can't seem to enroll in anything else. Every time I choose to audit another course, I keep ending up on the

I am enrolled in their(edX) probability course, but can't seem to enroll in anything else. Every time I choose to audit another course, I keep ending up on the page referring to the probability course. Is it a simple bug or are you not allowed to audit more than one course at a time?