6 months ago,my web site has shown on google in the second page for a good keyword .I frequently did lots of changes of the title,keywords and meta tags in the home page.Those days my website does not have good back links & good page rank. But now my site have good back links and the page rank is 2. yahoo,,,live,,msn and other of search engines show my site withing fist five results of that keyword. but the problem is google does not show my home page for that keyword. it show a sub page within 15 to 20 pages. i cant understand that,why google does not show my homepage. Is it a drawback of frequently changes of title , and meta tags?
yes could be... google doesn't like too many changes in the title and the metatags... stick to updating the content of your website instead.
Could be a probable reason. You should only change the title and meta tags only if you feel that they are irrelevant. If you already have a good title with keywords in it and ranking good then you should not disturb it. Now that you have already lost the SERP, i would suggest you to study your competitors site and make out that what are they doing better than you in terms of promoting their sites. And also become more aggressive towards your promotion efforts for your site.
Could be that you've got a lot of competition for your keywords. Are you building external links as well?