Why google decreased my indexed pages ?

Hi
Google has indexed more then 6K pages. But from last few days I noticed that Google has decreased the indexed pages. Now I have only 3K indexed pages. I have checked everywhere and see that there is no error. No crawl error, no sitemap errors...Just nothing. I don't know why its happened to me. Do you guys have any suggestion to increase the indexed page again ???

How are you checking number of pages indexed? Have you submitted sitemap and checked on the number of indexed pages on WMT?

Maybe deindexing duplicates created by categories, tags, author, etc...

Have you checked for errors in webmaster tools?

MaYbe you lost some backlinks ...