Check into sudden drops. If you see a sudden drop in the number of indexed pages, it might mean that your server is down or overloaded, or that Google is having trouble accessing your content.
Make note of unusually high index volume for your site. A high number of URLs could mean that your site has problems with canonicalization, duplicate content, or automatically generated pages, or that it has been hacked. In many cases, Google will send you a message when we detect problems with your site, so make sure to set your notification preferences.
Review sudden changes. Spikes or dips that appear in several charts can indicate problems with site configuration, redirects, or security.