You shouldn’t knock on their door the minute you can’t do something—obviously you’re expected to give it some proper thought first. But, if you do decide you want to see them, you might find it a surprisingly pleasant experience. As a student, I found this out much later than would have been ideal, when I went to see a professor toward the end of a masters course. He was very happy that someone was taking an interest in his subject, and he was willing to talk to me for ages.
