This is a shame - they really are good translators, but they lose a lot of business from us because we can't promise a client a job by a certain time as we just know they won't deliver on time. On the other hand, a nice Russian lady once called 2 days before a job was due. She was duly granted an extension after discussion with the client and is now top of my list for Russian. Most of the translation agency staff I know would call someone who's turned work down before they would call someone who's let them down on a job in some way. To be quite fair, the majority of translators are honest and will say if a deadline is difficult to meet - and if a job proves impossible to place because the client's deadlines are unreasonable, then the client has to rethink.
Communication is the most important thing.
A good way of forcing yourself to sit down and work out what you offer is to write your own brochure. It does not really matter whether or not you actually have it printed. Then the translation co-ordinator beats you down to a much lower price because you clearly are not wise in the ways of the translation world. If you think this way then you'll have a real problem sticking to your set prices. If the agency says it can't afford your rates then you have the choice of reducing them or turning down the work.