Quite some time ago I received the following letter from an author I had corresponded with and read a number of manuscripts from.
I am extremely frustrated by your repeated rejection letters. Each time I receive one I do the work you ask for and each time I submit revisions I get a new rejection. Clearly you are not making yourself very clear and explaining what you want. In one breath you encourage me and the next moment you tell me I'm not ready for an agent. Well do your job, make me ready. I want to learn and it's your job to tell me what needs to be changed and to make me ready.