Every now and then I go through my list of outstanding queries, fulls and partials. I am accumulating a pile of requests that have never been answered, despite the SASE that was sent along for a reply. Naturally, I would prefer to get a phone call - THE phone call, but all I hear is silence. Some day, I suppose, maybe years from now, the agent will uncover the gem in the pile of "Reject These" and get back to me, but for now, I can tally the request as lost and move on.
Twice I have followed up, and both agents have asked me to send the material again. One sent back a quick rejection via e-mail, the other never responded again. If the agent did not like the novel, that's fine, but the favor of a reply is appreciated. I have been sending out queries for a second novel, and, for fun, have queried a couple of agents who have been sitting on requests for the past nine to twelve months. Experiments can generate useful information - anything to inject a little fun into the grinding process.