You're at the same stage I'm at. If you haven't already, I'd suggest checking out queryshark.blogspot.com - that'll help you with querying agents.
If it doesn't mean that your query needs work, then it might mean that you need to look closer at the genres represented by the agents you're querying. Be sure to check if they accept the kind of work you're sending them.
Either they are kindly saying that your work isn't quite ready for publication or that it is too much like other submissions that they are seeing and it doesn't stand out from the large crowd.
OR
possibly they don't specialize in exactly what you write.
Hang in there, improve your craft, join writer's groups, and keep trying. It took me YEARS to finally start getting better responses from agents until I finally contracted with my agent who I admire and adore. Don't give up. Writing is a difficult business.