Same basic approach applies everywhere.
Don't forget to account for market share. If you're in an area where Bing has a lower market share, we may only be capable of delivery so much traffic.
If that's not the issue, then look to your content - is it thin? Is it largely duplicate?
...and the site: command does not return accurate data - hasn't for years. The features for seeing how your site is sitting in our index inside the Bing Webmaster Tools, however, are reliable in their counts. Check there for indexing volume issues first.