A recent thread regarding my sitemap to Google Webmaster Tools was closed so will start this one with a new question. Initially my current submission reads as: (DNS, Server Connectivity and Robots.txt fetch ALL GREEN), (791 queries with 32,926 impressions - 719 clicks), (13,846 URLs submitted -11,958 URLs indexed), (4226 images submitted - 3280 images indexed) and of this report in URL Errors: 0 server error, 1 Access Denied, 6 Not found. My question regarding the URLs indexed has me wondering…it was actually over 12,000 a few days ago but recently dropped to what it is in this current report and wonder why that is? Is there anything I ought to be paying attention to? This sitemap was actually submitted on or about February 6 (about a month and a half).Of all those images within, I’ve also tagged a majority with a specific keyword…
Google does not index everything on your site, just because it’s on a sitemap.
What is on all these pages? Are they all very unique? If a number of pages have similar content, it is possible only one of that number may by indexed. Sometimes things that have been indexed get dropped.
Unique content is important to indexing and SEO.
Indeed, search engines don’t always (or have to) index every page on a site, you have about 85% of them crawled so I’d say that’s not bad going considering how many pages you have.
It’s also worth bearing in mind that it can take months to get a lot of pages indexed. Some may never ever appear at all, search engines have to evaluate what pages may be useful and which ones won’t, then choose the most relevant ones to include in search queries.
Using a bit of Pareto then 20% of your pages will be viewed by 80% of your visitors, so long as those remain indexed then that’s half the battle won 
First Delate your Sitemap then again Submit your sitemap in google… I hope all is well again