As an agent who receives e-queries exclusively, I try my best to respond to all queries in a timely fashion yet admittedly I do get overwhelmed by the volume that floods my inbox every day.
It's very difficult to review each query carefully AND manage to get through a whole day's email submission pile in one or two sittings. I go blind after about ten or twenty e-queries; at that point I start to realize my ability to judge has started to wane and I have to walk away before I reject something that upon further inspection isn't so bad after all. Still, walking away isn't easy to do when I receive anywhere from 50 to 100 queries each day.
One way I've cut down on the number of e-queries I respond to is by refusing to respond to the ones that don't follow my submission guidelines. I'll overlook a mistake here or there, but if you're blanketing the agent universe with your "Dear Agent" letter and not bothering to appeal to me and my individual interests directly, then I'm going to guess you haven't taken me seriously enough - so why should I return the favor?
And if you haven't bothered to include a sample of your writing as instructed, then how am I to decipher whether you have the writing chops I'm looking for? You could have the best story idea, but if I'm not drawn to the writing then I'm not going to be able to sell it. But I need to be able to experience your writing in order to make that judgment call. (And no - the writing in your query letter does not count.)
Furthermore, if you've attached your sample chapters when my web site explicitly insists in big bold letters NO ATTACHMENTS PLEASE, then the response you get is my mouse cursor pouncing on the DELETE button while I'm cursing you for wasting my time and threatening my computer system with a potential virus.
As for requested material, I always respond though not always in a timely manner even though I try very hard to do so. It takes a lot of time and energy to carefully review a full-length submission all the while catering to the authors I already have in my stable. Agents are like day care workers trying to keep up with twenty-five children at once. Some need to be fed while others need their diapers to be changed. Some, thankfully, self-sufficiently go outside to play all day and you rarely hear a peep out of them unless it's truly urgent.
With all the many things that an agent has to do, the submission pile unfortunately doesn't always make it to the top of the priorities list. That might explain why you might get a response from me at 3:47 on a Sunday morning - or better yet 2 minutes after your e-query lands in my inbox. We agents usually know instantly whether something is not right for us, and also when there's an instant attraction to something. (Imagine George Clooney passing by Miss Snark's table at Michael's unexpectedly. How long do you think it would take for her to get him into her...um..."submission" pile?) (nanosecond)
What I hate is when I get a complaint from a writer because I've responded too quickly...as if I didn't really give that writer's work the time of day it deserved. This happens more often than you might think, and unfortunately makes me wish it was proper etiquette not to have to respond unless I really am interested. So, in that regard, I don't fault other agents who actively dismiss e-queries without bothering to send a rejection. Time is money. Spend it wisely.
Here's why you haven't heard back-1
Pulled from the comment column about non-responsive agents: