This has been bugging me for a few days now. I have a fairly new site and I haven't had any problems since now. It is a Wordpress blog. When I go to Google Webmaster Tools, I can see a thumb of my site that is outdated, also, the indexation speed has dropped and a around 30-40 of my pages are not indexed. I have checked the sitemap and Google shows no problems with it, and when I use the "Fetch as GoogleBot" option it doesn't show any problems, and after about 10 minutes my site gets updated, but after a few hours it gets back to the older thumbnail again. What could be the problem? Thanks.
Immediate indexing/ranking then sudden drop is normal for new sites, just continue building good links and contents to keep your site indexed more often..
Thanks for all the answers. Google did index my new posts but there is still that problem in the webmaster tools section. I guess it will fix itself over some time.
Add more content and then build links for your site. Google might not like your site also you should make sure that the content on the site. google likes site with a higher number of PR links as well as great content which is keyword rich.