Teaching Mathematics: the gulf between syntax and semantics

Why do students have trouble following an undergraduate mathematics lecture? Why are mathematical seminars frequently difficult to comprehend, even for seasoned researchers? Simplistic answers are that the speaker moves too fast, or assumes too much, or confuses the audience by being inaccurate. But in actual fact, slowing down, or devoting more time to background, or being more careful about accuracy, are in themselves no guarantee that the talk or lecture will become more comprehensible. There seem to be deeper issues at stake, which strike at the heart of what we understand by knowledge and its representation using language.