A lot of agencies have a firm rule against pitching to more than one agent simultaneously at the same agency. However - given that many agents never respond at all (and this is often a stated policy - "we only respond if interested"), shouldn't it be permissible to query other agents at the same agency after a certain period of time? How do you handle this?
I would think that if an agent thinks another agent at the same agency would be interested, they would pass that along to them. That's my best guess at least. It's been a while since I was in the query phase, but I don't believe I ever queried different agents at the same agency, and I do try to follow their policies. I did a lot of research to find the right agents to query, especially paying attention to what they have represented and what they had sold for those authors. I think that's really important when trying to attract an agent. So, I was always interested in specific agents, rather than an agency. My advice would be to move on to other agents at different agencies if that's their policy and enough time has passed where you infer they're not interested. Good luck with the search.
@deadrats has a very good point. You need to do your homework on the agents or publishers you are thinking of submitting to. I would also include a line in the query along the lines of, "If you think this would be a better fit with someone else in your agency, please pass it along to them."
Follow the instructions stated on the agency's site. Under their submission guidelines, this is almost always spelled right out for you. They'll say things like: Only submit to one agent at a time, but after X weeks/months, consider it a pass, then feel free to submit to another agent. Only submit to one agent total. If they think someone else is a better fit, they will pass it along. I have submitted to well over a hundred agents, and the most uncommon thing to encounter is an agency where they're fine with you submitting to multiple agents at the same time. There are certainly some out there, but not many. Why? Because most agencies only need one person to specialize in genres X, Y, Z and it's better not to double up and have agents competing with one another. Some of the big agencies have agents with overlapping manuscript wish lists, but most agencies are smaller or mid-size.