Take your pick from the following:

1. Your site is relatively new. Some search engines pick up and assess new sites faster than others. Suggestion: Wait a little and you may find the problem solves itself. If you're in a real hurry, talk to your SEO consultant.
2. Google, Yahoo! and Windows Live each use different and proprietary algorithms to assess the content of web pages. It's rare to see a site ranking the same on all search engines all the time. Thank goodness, too: the web is simply too diverse to benefit from any one fixed view of it.
3. You may have your site's robots.txt file and/or your meta tags set up incorrectly. (You'd be amazed how often we see this happen.)