Four reasons why Google might not use the anchor text in the links to your website

The text that is used in the links that point to your website has a major effect on the position of your website in Google's search results.

For example, if many people use the text "buy blue widgets" to link to your website, then it is very likely that the linked web page will get high rankings for the keyphrase "buy blue widgets" in Google's search results.

The link text (also called anchor text) is the text that is used in text links. Example:
this is the link text

Unfortunately, not all anchor texts will be used by Google. Check the following things to make sure that the links to your website pass the correct anchor tag:

1. The nofollow attribute
This is a no-brainer. Links to your website that use the rel="nofollow" attribute don't pass the link text to
Google. Example:
great keyword
 
You can use IBP to find out if the websites that link to you use the nofollow attribute: start IBP, click the "Links" button and click "Check links".

2. Invalid characters in the URL
If an URL contains invalid extra characters then chances are that search engines won't be able to index the link correctly. Example:
great keyword

In this example, there's a space at the end of the URL. Some webmasters found out that anchor text is not passed to Google if the link contains an extra space character.
Note that most browsers are able to correct this link and they will display the web page correctly. Unfortunately, search engine spiders seem to have more difficulty with malformed links (or they take them as a signal of low quality).

3. The links use 301 redirects
Google's Matt Cutts recently confirmed that Google won't consider all anchor texts that are used in 301 redirected links. Example:
great keyword

The web server redirects "http://www.example.com/page.htm" to "http://www.example.com" with a 301 redirect. In that case, it's likely that Google won't use the link text.

4. The first link passes the link text
If a page links twice to the same page then Google will use the first link text and discard the other link texts. Example:
This is an example. The link text great keyword will be ignored by Google.

The first and the second link go to the same URL. In this example, Google will use the link text of the first link, which is "This". The link text of the second link will be ignored by Google.
If the second link points to another page of the linked website, then both link texts will be used by Google:
This is an example. The link text great keyword will be ignored by Google.

Links are the most important factor when it comes to getting top 10 rankings on Google and other major search engines. Details about how to get links to your website can be found in the IBP manual (starting at page 91).

Article from Alexa.com