Can anyone help. I have a new site which I'm always adding new pages. I resubmit my sitemap to google about once a week and up until recently the 'Indexed URLs' has been going up. When I had 1000 pages, google had indexed about 400, when I had 1100, google went up to 990. I now have 1419 pages and now google's Indexed pages is going down. It's now at 850.
The crawler access is fine, the crawl errors only show 6 which I corrected a few weeks ago and there's only 37 HTML suggestions.
Submitting a sitemap helps Google discover urls. Sitemaps do not guarantee Google will index them or keep them in the index.
Google wants relevant & valuable pages in their index. To maike your pages relevant and more valuable make sure their is significant unique text on every page. It also will help if you have link juice flowing throughout the entire site.
ps I'm not sure if you are using Google Webmaster Central or doing a search on Google.com. If you are using a Google.com search you should be aware that the numbers ie "showing 1-10 of 850 results" are only estimated by Google. They are notoriously inaccurate. You should look at your analytics for accurate data to see what pages get traffic and what pages dont.
there isnt an average. it depends on your industry, your website and your content. i have worked on b2b websites that had product catalogs of over 100,000 items and it never had more than 10% indexed because the items were very similar to each other.
imagine trying to describe a significant and lengthy, keyword rich difference between a 1 inch gasket vs. 2 inch gasket. it wasnt exactly fun :)