I had my "perfect agent" in mind a few years ago. I sent a hardcopy query; never heard anything back. A few months later, I sent a follow-up by email. Never heard back. After that, I figured it was time to move on. There are still times when I wonder if I did everything I possibly could have to get my query in front of her, if I'd left any potential stone unturned. I would wonder if I, in fact, had the correct address, or if maybe another agent at the agency received that email and never forwarded it to her. Recently, I discovered that a published writer who visited our creative writing program while I was at MSU happened to have this particular agent whom I was courting, though I did not discover the fact until about two years after her visit. The irony is, I spent hours talking one on one to this writer because she had been told by my instructors what a "great" writer I am. Well, I could have kicked myself for not knowing (or should I say, perhaps, for not asking). And, of course, most published writers don't go around broadcasting that kind of information unless you do ask them. That might have been a perfect opportunity to make a personal connection with this agent, but I was unaware and so didn't take advantage of the opportunity.
However, unless another such godsent opportunity comes my way, I probably will not pester that agent again. Two queries is certainly enough.
I moved on and found someone who was interested.