Format consistency for your URL is very important. Be sure to use the exact same URL whenever you request a link. Even though a URL link beginning with "http" or "www" (or both) might resolve to the same page, Google sees them as different destinations.
If you want to see how many links you have, go back to Google and enter "Links: http://www.yourdomain.com" (without quotes). The inbound links for that domain will show up.
Mark the number of results down and try again without "www". Mark that number down as well. Now try it with the "www" but bot the "http://". Different amount of results will most likely show up. This is because Google sees them as different link destinations.