There could be several reasons a site is not ranking well in the search engines. Here are some possible reasons.

One is the navigation. Did your web designer include a java script or flash menu on the new site? Search engines do not like java script or flash menus. If the search engine “spider” or “bot” looking for new content discover a java script or flash menu, it will usually skip it and look for other content. When it skips the menu, it misses the rest of the website pages (unless a different navigation path is provided).

Another reason is a lack of content or content that is placed in images or Flash media. Some web designers love to do this. Google, Yahoo and MSN do not read the text imprinted on images or included in Flash movies. If a website has all of it’s text placed in images, it will be hard to find that content included in the search engine results. Make sure website content is in regular text.

Lack of incoming links. An incoming link is like a vote for a site. The more links, the more popular the site will appear to the search engines. It is incorrect to think that if I build a site, naturally it will be popular and rank on the first page of Google without any effort. Having other websites link to yours is a very crucial step to having the site rank well in the search engines.

This is a very simple overview. These search engine optimization issues are further discussed here.