It happens sometimes. Here are some of the common reasons you may not be indexed:

Showing up at the top of the major search engines can be easy or difficult depending on the competitiveness of the search term. This said, there are some basic steps that you can follow to make sure your web site shows up well for semi-competitive search terms.

Your link profile is potentially the most powerful aspect of your SEO efforts, especially in the eyes of Google. Quality counts over quantity, but it is important to get a good list of well-balanced links pointing to your site. Diversification really is the key. Try not to concentrate all of your efforts on gaining links from one source, and similarly try not to gain them using a single method. A number of tactics should be avoided wherever possible because they either offer you no benefit whatsoever or your page may be penalized.