I think that these moments might be where some of Williams' greatness lies. Tom's inability to enter the apartment reflects how he "doesn't fit" in the family. He has difficulty in entering because of his own emotional difficulties in committing to his family. As a character, he has one foot in and one foot out of the family, being there in physicality, but far from it in actuality. Tom is on the outside of the family, which is why it makes sense that he would have difficulty entering into the apartment. Tom lacks the "key" that enables entry, as he is unable to find his happiness as part of this family. For the most part, he struggles through this and does join the family, but it is a struggle and one that he can no longer fight at the end of the drama. His difficulty in entering is a small element, but it reveals more about his character and how Williams plays with a theory of correspondence in order to show something internal from something external. Since Tom is telling the story in reverie form, it makes sense that he has since experienced getting in anyone's apartment from a symbolic sense as he still is estranged from himself even when leaving. This helps Williams' point that there is no such thing as pure and total liberation. In the end, all of us struggle and have difficulty entering our own "apartments."