Office buildings in New York City are so large that many have their own zip code. Countless tenants occupy the spaces, a rabbit's warren of cubbyholes that masquerade as suites. Forget to include the literary agent's suite number in the address on the query, and chances are, that agent will never get your query.
So the first time you fire off your submission to Elizabeth Winick of McIntosh & Otis, you check the submission guidelines, stuff the envelope, and wait for six to eight weeks. The time passes. You hear nothing.
Could it be that you forget the suite number? No, they don't have a suite number in their address. Perhaps it was a typo. Easy enough to make a mistake in the zip code, and the letter could never have gotten delivered. What with cost cutting at the postal service, they might not bother to return the thing as undeliverable.
You query other agents. You write something else. Again, you think that Ms. Winick would be the perfect agent for your latest manuscript. The query gets sent, you wait six to eight weeks, and then nothing happens. No word.
Hard to imagine, but it's in the realm of possibility that you forget to enclose a SASE for her reply. No harm in trying again, this time double checking every single step of the process. You wait six to eight weeks. You realize that you will never hear from her unless she's interested.
No matter that you've enclosed the SASE for her reply. No e-mail queries accepted, but the "no response means no" that applies to e-mail will be brought in. Saves time and money, to not have to stuff the rejection letters into author-supplied reply envelopes.
Another step on the learning curve has been scaled. If they'd state on the website that Ms. Winick doesn't reply unless she wants more material, you'd save yourself a stamp, but you've learned that some travellers in an industry that deals with communication aren't great communicators.