The synopsis could very well be the problem, or part of the problem, since this is where a lot of writers take themselves out of consideration before an editor or an agent even looks at their manuscript.
Your question -- where online you might find advice on writing a synopsis -- comes up pretty often around here, and I don't think that anyone has been able to supply any useful links. You may have to look for what you need in some of the available books on writing.
Here are some titles from a recommended reading list put together by the agents Michael Larsen and Elizabeth Pomada, which I picked up at a large writers' conference they helped to organize. I don't know which of these have good sections on writing a synopsis, but if you run across any of these books you might want to check them out:
The Complete Guide to Writing Fiction , Barnaby Conrad, Writer's Digest Books
How to Get Happily Published: A Complete and Candid Guide, Judith Applebaum, HarperCollins
How to Write a Book Proposal, Michael Larsen, Writer's Digest Books
Literary Agents: What They Do, How They Do It, and How to Find and Work with the Right One for You, , revised and expanded, Michael Larsen, John Wiley & Sons