There are way too many reasons to go into completely, so I will ask you a few questions.
1) Is your domain relatively new? If yes it might be "sand boxed" meaning the engines are waiting to see if you are a spammer or flash in the pan.
2) Did you register your site with the big three (Google, Yahoo, Bing) see http://www.ucanseo.com/OffPageSEO/SiteSubmission.aspx for details on how to.
3) Are you on a FREE hosting account or a less than reputable web host (Hard to tell this one)
4) Do you have Quality text content that the can index, not flash or graphics?
5) Have you been compromised and your site hosts viruses and or malware?
6) Have you dumped your sites links into hundreds of link farms recently?
Good Luck.
webmaster@ucanseo.com
SEO doesn't have to be hard