I think that on these kind of sites expecting a reply is little naive. Most projects will get so many bids, they won't take the time to reply to them all. I don't have an account, but do they have some kind of skills page? Anything you can put there that will make them interested? Projects you've done, things to show off?
Well it's really up to the them. Just think about this: when you have like 50 bids on a project it's clear that you will only look at those who have a really nice profile, with lots of similar projects and very good reviews on the site. My advice for you will be to keep on biding, and try to mention similar projects you have done in the message. Try to write up your skills related to that project in the message and also try bidding a little bit lower than others until you get some projects and have some rating.