How long ago was your domain indexed?
It seems that Yahoo is in a transition period right now, they are transitioning to using the BING indexing servers. When you perform a search at Yahoo.com, it is using the BING Index servers. however, the BOSS API is still using the Yahoo.com indexing servers.
When you sign up to have your site indexed at Yahoo.com, it is being indexed by Bing, so the results cannot be returned by the BOSS API.
They are going to changing this in the BOSS V2 API which is slated to be released this quarter, we will put out an update the the BOSS extension once the V2 API is finalized.