That is very unusual because it really should be the other way around. You should be happy that your sites are listed in Google and just give it a little time and they will be in Bing as well.
I'm starting to see more hits from Bing in my search traffic but Google still rules in the visitor traffic stake
I think recently Bing.com has made big progress in indexing a site and it appears more accurate than Google, especially after the deal with Yahoo, IMO. I think a sitemap submission to Bing would help your sites in getting indexed.
Bing is a good source of traffic ONLY IF YOUR IN IT!! I have noticed some of my sites popping in and out of Bing its very annoying to say the least. I think Bing is doing what Google did in the past and slowly index sites for quality where as Google indexes you like mad then sandboxes you for a while or forever defendant on your niche after a manual inspection!