Hmm don't know but I do know that my site is the same way, maybe yahoo is picky... I'm in Google and the smaller ones but not yahoo.
Hmmm that is strange. You should be indexed by Yahoo if you have built a good amount of quality backlinks especially if you are indexed by Google.
Based on my experience, yahoo crawled my website slower than google. So, just be patience. keep build link, nofollow comment also improve yahoo crawler if you put an anchor link
I have a blog site but I don't know how it was indexed by Yahoo. I just found out later that my blog was indexed by Yahoo. I just social bookmark my blog to yahoo and maybe you can try social bookmarking. I don't know if social bookmarking in forums is available. Just try it and see what will happen, who knows.
Visit Yahoo! Site Explorer at siteexplorer.search.yahoo.com and verify your site with a meta tag or empty html file. From there, upload an XML sitemap that includes all of the URLs on your domain. You can also try getting involved in Yahoo! Search Submit Basic here: searchmarketing.yahoo.com/srchsb/ssb.php to help achieve more frequent and consistent indexing of your new content.
Yeah i agree, i usually don't even submit to yahoo and i get indexed within a few weeks because of getting some good related back links.. Just focus on back links for the three weeks so when it appears it indexes all your website..