I have a number of approved article in goarticles, articlerich, amazines, articlesnatch, articlesbase, etc , But i am not getting backlinks from these sites. What is the reasons behind it?
If they are do-follow sites which some of those are, you do have backlinks from them if you added anchor text to your author bio. These links aren't very effective, but the're good for diversifying your anchor text profile and building a ton of white hat links. PR links are what sky rocket rankings, but look unnatural if you have them alone. So if you purchase a PR4-5 link, use the exact keyword you want to rank for as anchor text for that backlink since it'll get you 90% of the results. Then create 10 articles, submit them to top directories using relevant anchor text for each one. These will get 20% of results, so you have 11 backlinks that will sustain a medium competition keyword ranking in most niches.
Hudders is right. High PR links are the name of the game now. Use low quality links like articles and social bookmarks to diversify anchor text. Use exact match anchors on the high PR rented links. It's working great for me now.