There are a few reasons to consider:

Have you recently removed the homepage content that we placed there for you from your website?  By removing the homepage content you removed not only keyword rich content, but you removed the keyword rich anchor text pointing to our optimized pages resulting in the search engines not being able to find our page as efficiently. To see results come back, we recommend adding your paragraph back on the homepage.

You could have chosen very competitive keyword phrases.  Highly competitive keywords could take significantly longer to rank in the search engines. Since you have a lot of competition for these particular keywords you will need to do additional work in order to increase your rankings. We would suggest that do some link building to help boost rankings faster.

Essentially, backlinks are the number of links pointing to your website from another website. This is very important to have since the search engines view these links as votes of confidence in your website. By having a high number of relevant backlinks the search engines will have trust in your site and will rank you high because of those links.

You could have very little backlinks.  Backlinks are the number of links pointing to your website from another website. It is very important to have good backlinks since the search engines view these links as votes of confidence in your website. By having a high number of relevant backlinks the search engines will have trust in your site and will rank you higher because of those links. Without having backlinks comparable to your competitors, it will be harder and take longer to rank in the search engines for your keywords since your competitors already have the trust factor by having those backlinks. Also, 50% of Google®'s algorithm is the amount and type of backlinks you have. This is why in addition to content on your website, that gaining backlinks is such an important part of search engine rankings.