When you are searching for an agent you must be prepared to send out multiple samples of your work. I had the same problem as you, I would send out the query letters but yet I would get no response, or I would get that ever so syntactical letter "We are sorry but your content is not what our agents are looking for at this present time".
This is when I learned one secret to the trade have two or three different query letters prepared and send them all out to various agents.
Also have a 30 second to a minute trailer made about your content and insert it on You-Tube and other various online resources (you never know who could be looking at your content).
When I first started looking for an agent I began by doing an internet search and realized there are so many resources and then this is when I found that by purchasing a subscription to an online writing resource community I gained posterity as distinguished writer.
Another suggestion is to purchase a website that introduces yourself as a writer/screenwriter. Give samples of your content on your website. This is also a good place that you can insert your trailers.
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