Many back links to providing increase your site to high PR .Just make many back link .For blog commenting and forum posting...Thanks...
You need to build new backlinks. Make sure they point to your homepage and make sure they have your keywords in the anchor text. This is key. If they don't, Google will penalize you. Even though you have some big backlinks, you don't have many overall (less than 500). How do you expect to compete with the top-10 sites in your niche who have thousands or millions? Start building links.
I have same matter with you in the past, when i search the keyword thiết kế logo in my country, at beginning, my site get PR 2 but suddenly my site disappeared for 1 month and it came back the PR 2 again in google search engine.
You just need few quality backlinks for your home page and your site will get PR in next PR updates for sure.
See the thing is not all back-links carrt equal weight... Of course, you need back-links from different sources, but just 2-3 powerful Pr4+ backlinks can do the magic! The point is that you need to try to keep building back-links consistently!
Maybe it's the cause of Google's Panda Update. I also suggest to get links from relevant websites with high PR, maybe that'll help you.
Writing content alone wont help you in the SERPs. You need good seo like linkbuilding done to you site.