A client's website is on the top of Google, MSN, Ask, etc. for a bunch of relevant keywords, but on Yahoo it's nowhere to be found - until you type the exact URL in to the search engine. Is Yahoo punishing my client? He use to heavily advertise on Yahoo which is the last time he had great search engine rankings for keywords. He stopped advertising to save money and was happy with the result of the other search engines - do they not like this?
Try using the siteexplorer tool by yahoo. This may give you some answers as what is crawled and when?