They don't typically index all of my sites, but the ratio is usually a little closer than that. They often won't index certain types of pages - are all those pages full content, unique pages? Have you kept out things like tags, or anything else that creates duplicate content?

Yes, Netmeg. I have no duplicate content. They are all unique pages.

Have you done the usual stuff like checked robots.txt? Checked WWW/non-WWW duplication? Checked WMT for crawl errors? Fetched as Google?

Yes, I've done all that, trebuchet. I'm really stymied.

One thing is that from what I can tell, that number of pages indexed that GSC reports isn't necessarily accurate. I have my sitemap broken out into various categories (event and taxonomy pages) , and the number of pages I get when I do a site: command for each category is usually higher (and sometimes substantially higher) than what's reported in Search Console.

This stuff is actually probably more for the Google SEO forum, but another thing you might take a look at in Search Console is any changes in your crawling history.

Most sitemap crawlers are highly aggressive in terms of what's submitted - I'd go as far as saying they almost always submit undesirable URLs.

The key question is which URLs are not indexed. The only good way to answer that is by comparing indexed URLs with submitted ones. This is not straightforward without specialist tools, however, if your website has distinct top-level directories, one "manual" way is to compare counts for site:example.com/directory with the count of URLs in your sitemap for that directory, and keep going for the biggest directories. For directories with big discrepancies, you can then drill down further to find the missing pages.

Is this a CMS or a hand coded site? How dynamic is it?

How does Bing compare? More pages indexed, fewer, or about the same?

It's a hand coded site. Bing has more pages indexed.

40,702 URLs submitted
....
It's a hand coded site.

Perhaps "hand coded" was the wrong either-or to use.

In fact, Bing has ALL my pages indexed!

If you're looking at the count for a site search, beware. Bing's estimated numbers are even less reliable than Google.

E.g.

• site:webmasterworld.com [bing.com] - 226,000 results
  
• site:webmasterworld.com webmasterworld [bing.com] - 42,300 results
  
• site:webmasterworld.com www [bing.com] - 231,000 results
  
• site:www.webmasterworld.com -webmasterworld [bing.com] - 530,000 results

Aside from this, I would usually assume that a much higher number of Bing URLs vs. Google indicates filtering of (perceived) lower quality content that Bing does not do very well at all.

Or, Bing does it better than g ... pretty sure the folks with (perceived) lower quality content websites would be happy with that. :)

Now the question is if Bing has them all, why does G not?