I have a website which has reached an Alexa rank of 250,000. It had four sub-domains and Alexa was ranking them as well.
Two weeks ago, my rank became 290,000 and Alexa now shows only one sub-domain in addition to the main domain.
I use the Alexa toolbar and all my friends use it. I would at least think that Alexa should rank a sub-domain which we use everyday. Why did this happen?