So I have recently started a handful of niche contant sites (Garden, Heath, Travel ext...) I have decided to focus my main efforts on three of them. These site are the ones I have back linked and developed farther then just slapping together a site with a CMS and some cheap paid for contant. Now here is whats confusing me, I have ten sites in total, seven of them have been indexed and crawled by Bing. The three that I have put the most into have not ever been crawled by Bing. Its been some weeks since my other sites were first crawled and they all link to each other. I have looked at the Bing web-masters tools and search their site for information but still have no idea why three of my sites have never been indexed and seven have. Google, Yahoo and Yandex all had no problem indexing the site.
Please be patient. Did you ever optimize your site for MSN, the old Bing? This is pretty common for unreliable bots activities. I would recommend you just look over this issue temporarily and concentrate on current link building process. I am sure the rest 3 of your sites would be indexed by Bing in a soon. P.S. You may consider to check if .htaccess or robots.txt has something against for Bing or not.