Why some posts are not rank well?

[NiteRider]

I have a smart phone related blog. My blog has PR 4 and Alexa rank 95,000-110,000. I have a issue with some blog posts. My blog all the Sony Xperia phone related posts are well rank and they generate high traffic. But all the Samsung, HTC phone related posts are not in the first 100 search result. After published new post, I bookmark them in several social bookmarking sites. What is the reason for that? How do I improve my ranking?

Thanks!

 

[Kristinakathy]

May be it depend upon the demand of users.

 

[NiteRider]

i don't think