This is a complicated question. I’ll do my best to address it, but keep in mind that there are many variables to querying agents.
First of all, check the agency website for updates and their latest formal guidelines. They might have gone on leave. They might have switched agencies. And most likely, they may have submission guidelines that state how they only respond to submissions if interested. (You see this a lot with a line such as, “If you don’t hear from us in 8 weeks, it means we are regretfully not interested in your project.”) So keep in mind there might be a very good reason as to why you shouldn’t follow up or rather why you shouldn’t follow up right now.
Just a note: if an agent makes it sound like they do indeed respond to submissions but they don’t have a time frame for their reply, I say follow up after three months.
That aside, let’s say an agent claims they respond to submissions “within three months” and it’s been three and a half months with no reply. A few weeks have passed since the “deadline,” so now it’s time to nicely follow up. All you do is paste your original query into a new email and send it to the agent with a note above the query that says something like: “Dear [agent], I sent my query below to you [length of time] ago and haven’t heard anything. I’m afraid my original note got lost in a spam filter, so I am pasting it below in the hopes that you are still reviewing queries and open to new clients. Thank you for considering my submission! Sincerely, [name].”
That’s it. Be polite and simply resubmit.
Before you send that precious follow-up, make darn sure you are not to blame for getting no reply. Perhaps your previous email had an attachment when the agent warned “No attachments.” Perhaps your previous email did not put “Query” in the subject line even though the agent requested just that. Or perhaps your previous email misspelled the agent’s email address and the query truly got lost in cyberspace. In other words, double-check everything.
And if you send that follow-up perfectly and the agent still doesn’t reply? Then forget ‘em! Move on.