I think you need to get more to read or gets even more knowledge for Google indexing tactics. I had had the same problem. You can take better suggestions from an SEO consultant.
The one thing I have learned is that you should never put unnatural backlinks on other people's sites without their approval.
Google keeps an eye out for "the type" of links pointing to your site, and if they have gone up dramatically all of a sudden, or come from sites that wouldn't normally link to you (because of their subject matter etc..) then they will put a penalty on your site for trying to game their system.