Why does the Alexa ranking matter? Surely far more important measures of the success (or otherwise) of the site are the level of traffic it’s attracting and the conversion rate.
Unless you are using the paid version of Alexa, then the ranking will only record visits by people with the Alexa toolbar installed, which is not necessarily a high percentage of your visitors - or a representative sample.
We already have many topics asking about Alexa rankings. Have you read any of them? You might find some helpful information there.
For example:
http://www.sitepoint.com/community/t/alexa-ranking/93128
A quick search of the forums will find many more.