It has never been a literary agent's job (nor do they have the time) to give writers a page-by-page analysis of what is working or not working in a manuscript or book proposal. So, they send out various forms of rejection letters. These letters (while necessary) do not give you any idea what the real reasons are for the rejection or how to fix it.
Book publishers operate in the same manner. When I was acquiring projects as a Random House Senior Editor, I received at least 60 packages per day. First, I sorted them. The unagented manuscripts went to my assistant for a first reading. I never saw most of them again because they were simply not publishable. I read enough of the agented submissions to decide whether I wanted to see more of the writer's work or not.
Because of the recession, agents and publishers are buying fewer projects. They are also uninterested in taking on projects that require a great deal of work......unless the writer is a celebrity.