As mentioned earlier, mobile searches are more than desktop searches now. And Google is experimenting mobile first indexing and will start considering page speed as a ranking factor by July 2018. You should improve page speed of your website .
Site structure, internal link structure, looped redirects, unsupported scripts sometimes affect crawlability. So you should keep checking crawlability from time to time.
For maximum benefits, you should get all of your website pages indexed. But sometimes, Google removes web pages from Google index. You should keep checking indexing of your web pages.
