This could be for a number of reasons, but one common reason is because the domain name was previously expired.

Google ignores expired domain names, this is because at one time webmasters would go around and buy expired domains for the PageRank that was on that domain in the hope they could redirect the PageRank to their own domains in order to fool Google into believing their website was more popular than it really was.

Google put a stop to this by no longer automatically indexing expired domains, however, there is a way around it.

Re-inclusion Request

If you find that you have recently bought an expired domain name and are struggling to get your website into Googles index, then you need to submit a Reinclusion request.

The correct process is as follows:

Go to: http://www.google.com/support/bin/request.py

Select - "I am a webmaster enquiring about my website"

Then select "Adding my site to Google", then submit

In the title of the form ensure you put "Reinclusion request for expired domain"

Then write a quick explanation regarding being the new owner of the domain, including the domain URL.

Once you have submitted the form, you should receive an auto response email offering you links to help pages, these obviously can't solve your problem, so you will need to reply to the auto response email and explain that you still need their help.

Once you have replied to the auto response email, you will eventually receive an email informing you that your request has been forwarded to an engineer (If there is nothing you need to do yourself).

This is the point that you know Google are on the case. You should find after a few days that you site begins the normal process of being included in the index.

To find out if your domain name is expired, go to a website called The Wayback Machine. You can try out your URL to check if your domain name was previously used before you bought it.

As long as you inform Google that you are the new owner of the domain, and you correctly submit a Reinclusion request, you should have no problems getting your site indexed into Google.