It has been observed that it takes about 6 to 8 months for a new site to be in Google’s index for targeted keywords. So, if your site is new then Google may not show you in search results for your targeted keywords, sometimes even for your business name. You may also not see your deserved Google Page Rank. In this period, it is said that the site is in Google Sandbox. This is just an observation and not a confirmed theory.

So, if you face this problem after submitting your site to Google, don’t get disappointed. Instead add more content and increase your link popularity. Those will help you to hit the top when you are out of it. This has not been heavily observed for Yahoo or MSN and you will get what you deserve from them along the way.

In addition to being new, over-optimization can also be a reason. SEOmoz has a detailed analysis of Google Sandbox.