Agents role/Casting process

I was wondering what is exactly the role of an agent and what do they do to secure work? I have been with one now for about 8 months and have just received 2 TV castings from them as well as 1 TV commercial casting (They are actually a very biggish agent and very well known throughout the industry and represent a lot of very well known actors). They are very nice but when I call and ask whats going on they always say that I have never been requested for jobs that are being cast for. Is it an agents job to push i.e. sway casting directors to see actors even though they may not be right for the role? I understand that when I get wind of something that's being cast I get all excited and ask them if they can put me forward for the jobs and my agency replies that the casting director knows me,as I have met them before and then says if they (the casting director) think I am right I will be called in to audition. Is this the way it is with all agencies or is my agent being lazy? I was also wondering how Casting directors go about choosing the lead roles, do they have an open casting to see who auditions the best or do they have well known actors that have a track record of credits that they choose from? Is there a frequency I should expect with regards to the amount of castings I should be going on? I am pretty new to this working with a legitimate agency- the last one I was with charged me $400 for pictures etc but now I am a little wiser and have a real agency that doesnt charge that. Any advice would be great.