This question was posted on a popular actors forum, and I posted an answer there that I wanted to repost here:
“I like my agent- he is my first and has a good reputation. However I have had zero auditions through them. I do keep busy, but all on my own. I have been with them for over 4 months. I did send a decent letter and updated him to my activities. I do this often. I made the point that I was a senior actor and cannot wait 10 years to get established. No response as yet. Is it time for a change?”
This might go without saying, but have you tried talking with your agent and discussing your concerns? There may be a lot going on behind the scenes that you are not aware of, or you may be right- your agent may not be the right fit for your needs right now. Either way, I think you'll get a better picture of what is going on once you talk to him.
When you talk, it would be a good idea to discuss what your expectations have been over the past few months and what you would like to see happen in the future, and see what he says. Most likely you'll find a divide between your two perspectives and you can iron things out from there. If he agrees that you'd be better served by another agent, perhaps he will have a referral of who to try next? That may be a Utopian view on my part, but this agent saw something in you when he signed you, so asking him for supporting in finding a better fit is a great way to make sure no bridges are burned.
Again, you may have already done this but I wanted to be sure to mention it just in case. Whatever happens, I wanted to applaud you for getting signed, and being so successful in getting yourself out there!