It takes some time to get indexed after you have submitted the sitemap to the webmasters tool.

I have the same issue. I have submitted our website more than 1 year ago. So, please give the exact reason.

Sometimes, Webmaster tools showed old data. First of all you need to check the sitemap either it is working fine?
If everything is working fine then re-submit in Google webmaster tools.
Otherwise generate new sitemap and then added it back.

Firstly you look Google Webmaster Tools dashboard. Forget about all the other tools available for a second. If Google sees problems with your website, then those are the ones you will want to address first. If there are problems, the dashboard will show you the error messages.