My ecommerce site has about 4,000 pages. At the end of February, GWT had about 1,700 pages indexed. A couple weeks later, it dropped to 750 pages indexed. When I saw that I discovered that my sitemap was not working, so I put up a new one and submitted it to Google. Google acknowledged that I had submitted 4,150 pages but indicated that it only indexed 450. My latest number is 191 pages indexed as of today. Obviously, something is going wrong, but I have not made any changes to the website in this timeframe. Can anyone provide guidance as to what may be going on? Thanks.
Probably this is due to increased no of not found urls. Make sure your robots.txt is not disallowing any urls. Make sure you have not made any changes to .htaccess file which has internally changed any allowed url structures and Google is seeing the indexed urls as not found. attach your GWT errors pages screenshots here to analyse it
Another possible reason can be google might not be finding your website useful enough. Try Updating your content and offers