I really don't know. I'd say that addressing the fact that you feel like they have no substance is probably the first step. Concentrate on making them feel more rounded, and you'll hopefully start liking them more. I can't give you any specific help with that since I don't know your characters, but if you can recognize the problem, that's already very good I'm sure you can figure out what to do to at least some degree.
Also, don't put too much focus on trying to make your characters likeable. Sure, that's good to some degree, but some people just won't like those characters and there's nothing you can do about it. Besides, there are lots of characters who would be awful and almost universally hated in real life, but because they're fictional, they can be popular, well-loved characters, and there are also people who enjoy things even though they dislike the characters. I generally prefer to focus on making the characters convincing and rounded, and then to develop them some more to make them more likeable if it's needed rather than trying to build "likeable" characters from scratch.
If you want me to clarify or elaborate on anything, or if you want more specific help and advice, feel free to ask!
This message was edited by the author on June 11, 2012 at 12:16:06 PM