My PR 4 site have been enjoying top pages in Google and Yahoo for almost 1 year. Just this last week i found out that i no longer see any page in yahoo.com I do not optimize in yahoo since my priority is google, yet some traffics are good from yahoo. What happen when you can no longer find any page from yahoo result?
Get more links. This can happen when your keywords get poached. The secret is to just build one link per day for your main key word, but you can never stop
I forget to mention, what happen if this is the case site:http://domainname.com and you will not find single page in yahoo.