It seems that to get a break you need to be in the right place at the right time or offer work at very cheap rates or hope that the last CC didnt do such a good job and hopefully get a foot in the door.
You've just about got it sussed in that one sentence!
That obviously applies to those driven by price, which constitutes the majority unfortunately.
All you can do is recognise the ones who appreciate the quality angle (but still not too expensive!) and look after them really well.
I was down to one agent until recently when another one has started putting work my way. I have shed no tears for the ones I've lost.
It can be an infuriating and thankless marketplace to trade in, but you will find your niche, jsut think it through and bear in mind you need to adapt to what they want and not the other way round

p.s. I would make personal calls to the offices, with a covering letter in hand. Get the name(s) of the relevant people without being pushy and then follow up with a letter and then a call.