That's a good professor. I think everyone who has dabbled in Philosophy, especially of the ancients, usually Plato, has had that experience of thinking they are smarter than Plato and that he is making obvious errors all over the place. The first thing any student of philosophy (or a lot of other fields, really) has to do is to cultivate epistemic humility; he must train his instinctive reaction to "I don't get it" or "this is obvious" away from "therefore the author is dumb" and towards "I must be missing something".

That isn't to say that every author and philosopher is infallible and you must assume they are smarter than you at all times, but just that you need to acknowledge that very famous works are typically very famous because, in the opinion of very learned people, they contain depths to be plumbed. If one isn't seeing those depths, he needs to look harder.