Reasons for non-inclusion: There are a couple of reasons why the website does not find a place in Google's index. One could be because of the waiting time even if the web master has submitted the request for inclusion, most of the time, he cannot do anything but to wait for the inclusion. The next reason could be because of some violations on the website which forces Google not to consider including the website again – Still Google won't tell you exactly what the problem is, most of the time. Remedial measures need to be taken immediately to solve this problem and sometime you don't have any chance to find the problems by yourself but you sure can use SEO experts to check your web-site.
Reasons for exclusion: There are many reasons why Google should exclude a given website from its index. Some of them are over-optimization, spamming, illegal content, inter-linking, and hidden text. Over optimization of a website could lead to suspicion and hence Google excludes such sites.
How to get re-included: Once the web master finds that his site has been excluded by Google, he must find out the reason for the same. Once the reason is found out and corrected, he must apply to Google for re-inclusion. The instructions and explanations need to be carefully studied and complied with before submitting the request since sometimes getting representative attention twice is more difficult. Other than this, nothing can be done except wait for reply from Google.
Tips: There are a few tips which can be used for getting re-included in Google index. The web master can own up the errors and rectify them as Google will never consider the request for re-inclusion unless the errors have been fixed. He cannot afford to be rude to Google for the exclusion as Google is not obliged to include the website for free traffic. The guidelines should be studied thoroughly and complied with. Double checking is always better and surely recommended. He needs to wait for the reply from Google as it may take some time due to the volume of emails that Google receives. If the site has been banned for the first time, there is some hope that it may be included but if it has happened many times, then the chances are very bleak. He should assure Google that the mistakes will not occur again and to take better care.