I don't have an agent at the moment, but I am querying right now. I was also querying about 2 years ago, before I decided to re-write the book that I am now sending out into the world again. Response times are much, much slower than they were even two years ago. (And when I signed with my former agent six years ago, it only took a couple weeks, start to finish.) It's hard to send something so personal as a book out there, get an enthusiastic request, and then wait months to hear back. It's probably even harder to wait that long when you've already signed with an agent, and your agent is supposed to be as excited about your new work as you are. But I think the business is just really slow right now. Maybe because more and more people are writing for children, maybe because agents have bigger and bigger lists... I think there may also be many more types of rights to manage, with the advent of e-books, and a lot of backlist to consider what to do with, now that the market is changing.
That said, I don't think it would be a bad thing to check in with how your agent is feeling about your project, turnaround times, and whether or not there's anything exceptional about their current workload queue. Consider saying something like, "You work very hard for me and it's obvious that you have a lot on your plate, but I am feeling anxious about how long responses are taking compared to how they were a few years ago. Could you give me a realistic idea for how long this process takes now, with all these changes in the publishing business?" Your concerns matter. Don't swallow them, but do find a way to address them without alienating your agent. If you continue to be ignored or pushed to the bottom of the list after the conversation, then it might be time to decide if you want to live with the relationship in the state that it's in.