Most writers will say that one of the most difficult things about querying is the waiting period.
The primary reason an agent has not responded is simple, a lack of time. They probably have not had the time to read the cover letter or manuscript.
Ideally, a query would arrive, an agent would read it, request the full manuscript, and then the next day offer representation. However, most of an agent’s day is spent taking care of existing clients, and we almost never have the time to read unsolicited material in the office.
Client deliveries will always come first, and so unsolicited projects fall to the bottom of the to-do list.
We receive around 50-100 queries per week, and even if it’s only a cover letter and sample chapters, it takes time to read material. It’s usually the queries that seem promising, the ones that we’re not sure about that take the most time to consider.
If an agent has requested a full manuscript, this is a great sign. If an agent has not responded after 6 weeks of requesting a full manuscript, it’s fair to send a gentle nudge to an agent to ask whether they’ve had the chance to read it.
It’s also true that agents sometimes don’t respond because they haven’t made up their mind.